Well since Yukimura's partnered up now:

[@Ijoyen]

For being summoned in Staging Ground, the following benefits are accorded. 

-Rank-up to MGI (D -> C).
-Rank-up to LCK (C -> B).
-Addition of a plus modifier to STR (B -> B+).
-Acquisition of Personal Skill: 
[quote]Prana Burst (Flames): C+
The body and weapons may be infused with magical energy, using instantaneous release to elevate one's ability. In Lancer's case, burning flames turn into magical energy that infuses her spear. During use, the plus modifier on STR is active; in all other cases it is treated as inactive. However, the prana efficiency is horrid. Using it once is enough to deplete a Servant with E rank MGI almost completely.
Less a result of the Servant's abilities and more a form of inherited capability from the Noble Phantasm "Crimson Demon of War", a means of tapping into part of the Noble Phantasm's power without directly invoking it. While the Noble Phantasm is itself active, prana efficiency improves immensely and this skill is treated as continuously active, though this does not mitigate the cost of the Noble Phantasm itself.[/quote]