It's highly debatable if it is an advantage, many think not. There have been a couple of threads... search under 'Daimyo'.
Well generally you wouldn't want your vassals to 'obtain' one. Better to get it yourself then give it to them even if you want them to have it.
There can be more complex reasons, I don't feel I know the game well enough to say yet but -
1) you get 3 honour for giving away a title (daimyo or kokujin) but you do not lose any income when giving away daimyo
2) your daimyo can recruit more retinue (which can be an advantage but can also be a pain if you want to capture all the provinces yourself)
3) your vassals can then give away kori's (debatable as well whether you generally want this but you may)
The one situation most players agree it is worth giving away a daimyo title is to your heir.