It may be useful to find out if you have a short luteinising period, but basically it's just a way of feeling that you're doing everything you can.
Me & DH have been trying since Jan 2005, and I started off measuring my BBT with a Boots kit. The thing is, though, tiny things affect it and make it invalid, like drinking the night before (even 1 unit), having a cold, or forgetting and getting out of bed before you take your temperature. Personally I found it very frustrating, and my graphs ended up looking like the Andes.
I then started using Boots ovulation testing sticks (like pregnancy tests). They're much trickier than they appear, and it's very easy to get a false reading.
I understand why people buy these things: it's for the same reason we come to sites like this one. You can't stop thinking about TTC, and so you do these things as a sort of 'constructive' time-filler.
We don't use any of those things any more. I've stopped drinking alcohol completely, I'm at a healthy weight, I eat very well and I do plenty of exercise. This has made it easier for me to listen to my body, and hopefully have sex around the right time.
I don't think they're a total rip-off, but they're certainly not magical panaceas either.