Just a couple weeks ago I was teaching a handgun course and there came a point on the range where I realized, looking down the line of students, that every one of them was wearing / carrying their handgun differently. There is absolutely nothing wrong with that, but the follow on was me examining each platform trying to ascertain what specific purpose it had been built for. Obviously it had to carry the handgun securely but one guy had two magazine pouches also mounted; another had a flashlight and a knife; another yet had a grenade pouch on his... things that make you go "hmmm..." So, with those thoughts in mind, I felt it would be good to discuss a few options and offer up a few suggestions for what they're worth.
Understand up front that I'm only talking about "tactical" platforms. Of course, they were deemed "tactical" because, back in the day, they were most often used by SWAT folks. Since then we've (shooters) come to realize that all of the characteristics that make them great for use by SWAT guys and soldiers also makes them great for use by we "commoners". (I'm not a SWAT guy anymore) What makes them so great?
First off, they position the handgun down off your hip. For uniform use that may not be optimal (and your agency may not allow it), but when you think about drawing your sidearm, it's far easier to get it up and out of a holster that's on your thigh as compared to the holster that's up on your hip. Further, depending on whether you use a low-ride, mid-ride or high-ride mount for your hip-level duty holster, you may have to almost stick your elbow in your ear to get your gun out. Also, if you've ever tried to draw your handgun while seated in a vehicle (from that hip-level holster) you've realized it's not as easy as the movies make it seem. With the weapon down in a thigh-mounted holster you can move it around a little on your leg so that it's actually closer to the top of your thigh as you're seated - usually making it easier to get to.
Secondly, by moving the holster down onto the platform, you free up some landscape on your duty / gun belt. Depending on how much gear you're trying to put on that belt, having an extra two to four inches might make all the difference in the world.
Finally, since the platform that the holster mounts on is wider than the holster itself and (typically) at least as tall as the holster is with the weapon included, you have some space onto which you can mount other items. That is, of course, assuming that the manufacturer of the platform you're using thought of that and gave you any space to work with - or a method for doing so. What you use that space for (and how) is the topic of this article.
Now, many a manufacturer makes a "tactical platform". Quite a few of the traditional holster makers in the law enforcement market altered a duty holster by putting it on a thigh platform so that they could add a "tactical holster" to their product line. The manufacturers who actually gave it more thought than increased sales saw the opportunity to ALSO sell some accessories IF they provided space and a means to mount those accessories to the newly available platform space.
The first attempt by some companies was to provide thigh straps that were of sufficient size and stiffness to carry pouches normally worn on a gun belt. While that worked (and still does), it tends to make the thigh straps a tad uncomfortable unless they're well broken in - and then they tend to bend and warp so that the pouches aren't necessarily all held where and how you place them.
The second attempt (also still in use by some) was to mount pouches to the holster body itself. This worked (and still does) pretty well with nylon holsters and the utility pocket usually fits a spare magazine as well as some flashlights or knives.