What's the benefit of using leather grip ?
 

Just like the title says. I am using stock base grip right now.

Leather grips have a different feel to them that is liked by some, but not by others. It adds weight to the handle. I wouldn't say there is any "real benefits" to a leather grip other than the different feel and weight to the handle. If you've never used one, you should try it and see if you like it.

I like leather grips because they're firm and give me a lot of feedback. They tend to get slippery so I add an absorbent overgrip and I run through reels of Tournagrip like theres no tomorrow.

The problem with leather grips is you have to take care of them and allow them a breaking in period. They tend to shrink over time, but also get better feeling over time and last for the entire life of the racket unlike other overgrips that fall apart.

It will take practice to regrip a racket with a leather grip because you will probably not get it right the first time. The spacing is critical.

Try it, you'll like it.
 

I use Babolat leather grips with Babolat Pro Team overgrips; an unexpected benefit I noticed is that leather grips added power to every shot. I can't explain why but it's there. There is also the firm or tight feel leather provides that is better than what other grips offer.

However leather w/out the use of an overgrip will become very slippery and frankly not very comfortable, in my opinion. So the prime combination is leather w/your overgrip of choice.

I think they get me a more "vivid" - opposed to mute - feel and, of course, there they are: the bevels. Can't play without leather + og, but I recognise they're not much comfortable like a synthetic replacement.

+1 for the bevels.

All personal preference, just like every other aspect of your frame

Leather:
Heavier than most synthetics
feel bevels more
firmer
doesn't break down like a synthetic grip, usually plays a little better when it's broken in (IMO)

For me the firmer and bevel advantages are the main reasons to have leather grip. The first allow you to have looser grip and have the racquet swings more freely, while the second is very critical to 1 HBH - I believe every kind of stoke is also helped.

Anyway, to avoid blister you still can add overgrip and still have the benefit of the leather.

Yeah, my synthetic grip started to lose its shape so I switched to leather. Like others have said, firmer feel, bevels etc. The extra weight (not too much difference but enough) made my racquet more headlight.

Yes it's an easy way to add some weight for customizing and IMO offers more traditional feel to frames