My sister and I have a running joke that our brother watches nothing but Law and Order and that he watches it all day long. The joke is a bit of exaggeration, but the truth is that it is not that much of an exaggeration. Law and Order and its spinoffs do air an inordinate amount of time during the day and on several different channels at that.
Consider this. Law and Order airs up to six times a day on TNT and airs most days of the week on that channel. Its spinoff, Law and Order: Special Victims Unit, airs up to three times a day on the USA Network, sometimes more, and also airs most days of the week. Law and Order: Criminal Intent also airs up to three times a day on the USA Network, on several days of the week. Both TNT and the USA Network show marathons of Law and Order, Law and Order: Special Victims Unit, and Law and Order: Criminal Intent. None of this is counting the new episodes of Law and Order: Special Victims Unit airing on NBC, the new episodes of Law and Order: Criminal Intent airing on the USA Network, or the new episodes of Law and Order that will air on NBC later this season. Nor is it counting the local stations on which those shows might air (for instance, here KZOU shows Law and Order: Criminal Intent). Of course, there are other shows that air frequently on other cable channels (CSI : Crime Scene Investigation and its spinoffs are an example--airing on both Spike and A&E), but I swear that Law and Order and its spinoffs air even more than CSI does.
As to why TNT and the USA Network show the series in the Law and Order franchise so often, I would guess that there are some very basic reasons. The first is that the Law and Order series are still popular. Never mind that the original Law and Order was almost cancelled last season and that Law and Order: Criminal Intent moved from NBC to the USA Network. Reruns of the shows still do very well in the ratings. Regardless of how they perform on NBC in primetime, the Law and Order shows still have a loyal following.
The second reason is that the original Law and Order has run so long. Law and Order is currently the longest running network drama series, second only to Gunsmoke. As of now it has run 17 years, with 393 episodes. This means that a cable channel or local TV channel, even running the show several times a day, could show the series without repeating an episode for quite some time. Indeed, if a station just showed Law and Order five times a week, it would take nearly a year and a half to show the entire series. Its spinoffs have had good runs as well. Law and Order: Special Victims Unit has currently run for eight seasons. Law and Order: Criminal Intent has run for a total of six seasons. In both instances, a cable channel or local station could show either series for quite some time without repeating episodes. The long runs of the shows in the Law and Order franchise makes them very attractive to programmers, who generally prefer series that they can show with a minimum of repetition.
The third reason is purely a matter of my opinion. Quite frankly, there have not been too many good, hour long dramas on the networks that have had long runs. As far as I am concerned, ER jumped the shark sometime after its fifth season. The shows in the CSI franchise have generally been good, but they have not ran nearly that long. The same holds true for Gilmore Girls There have been some very good genre shows in the past many years, such as The X-Files, Buffy the Vampire Slayer, Angel, and Alias, but they generally become fodder for local stations on Saturday night. I would guess this makes them less attractive to cable channels, who won't necessarily want to repeat a show that many,many local stations are showing. The shows in the Law and Order franchise have the advantage of being quality series and having had long runs. They are certainly better than much of what airs on network television.
The fourth reason goes hand in hand with the third reason. It seems to me that beginning in the mid-Nineties the cable channels sought out newer and newer shows to fill their schedules. Prior to that time it was not unusual to see shows made in the Fifties and Sixties on the various cable channels: Perry Mason, The Wild Wild West, The Avengers, Bonanza, and so on. Around the mid-Nineties, however, all of this changed. The cable channels started showing series that the networks had recently cancelled or were still running on the networks. Shows like ER and Law and Order began to push out the older shows. The problem is that there are fewer, quality new shows than there are good, older shows (let's face it, it is easier to find good shows when one uses the whole history of television as opposed to the last ten years). As a result, the cable channels will natural tend to show the popular, quality shows over and over again. This is why TNT and the USA Network show the Law and Order series so much and why A&E and Spike show the CSI series too much.
As to whether this is a good thing or not, I submit this is a matter of opinion. I love the Law and Order series myself (particularly Law and Order: Criminal Intent), although I don't want to watch them all the time. Personally, rather than showing Law and Order and its spinoffs constantly, I would rather TNT and the USA Network pick up some of the older shows. I honestly think that shows like Kung Fu, The Man From U.N.C.L.E., and Peter Gunn still have an audience. At any rate, they would certainly be a change of pace.