If you speak with elder Lyons he will tell you to speak with scribe Jameson. You can find her in the archives during daytime hours. Speak with her and she will give you the deal behind them. So the big question is what do the holotags do?
Well the first thing you get from them is caps and XP. You get 200 caps and 25XP for each holotags you locate and bring back to Jameson at the Brotherhood Of Steel Cidatel.
Its generally not worth your time to go out and look for Brotherhood holotags, but if you do find them pick them up. Once you have enough it's worth the money to head back and hand them in for some easy caps.