It reveals new heroes and monsters basically, at a higher level.  So, if there are still some weaker heroes hanging about, it may just be from a previous tech that you learned. 20% food and mining is pretty awesome in the beginning, but if you've managed to get to those later Adventure techs, they aren't all that necessary, since you're probably going to win

How are their stats? I know when I've researched those techs, that it seems like the base stats of the heroes have improved quite a bit.

I don't know for certain, but I think the heroes just have one trait, and stats that are random.  Ideally, you would want someone with bonuses to mining/farming/production to have a high charisma, so you can just plop them in a city and get their trait bonus plus the prestige bonus associated with their charisma, but I don't think it balances it out like that.  You can have some great stats for warriors on a hero that provides a production bonus, so you just have to sort of pick which way to go with that hero.

Honestly, almost any hero decked out in legendary armor and weapons becomes a force.  You just have to level them up and boost their Strength and Constitution for them to be even better. I never boost my +mining/food/production heroes though, just stick them in the cities for their traits.