What's been said about goretex is true, however I think it's important to look after your boots, leathers and gloves by using some sort of polish or leather nourishing treatment on a regular basis. Bike gear is not cheap, especially if it's leather, so it makes sense to me to look after it.
It should still allow the leather to breath and it should leave it more supple and lasting longer. The leather's water/dirt repellency should also be boosted but as somebody said already the primary barrier to water is the liner, goretex or similar. If it blocked the leather, everybody with polished shoes would have sweaty, stinking feet and nobody would bother!
I can vouch for Renapur, used it for years on various leather items incl bike stuff and have heard good reports about the Altberg stuff, which I believe is pretty similar.