Yes, there is a bonus based on the offensive and defensive values of the group leader (5% per point I think). And besides the group units will benefit from any special ability the leader has.
So what arsan is explaining is you get the leader's combat bonuses which are built in, 1 in offensive or 2 in defensive, for instance. In addition, if they have some special combat value like siege or supply value like forager this applies to all troops they command.
Another benefit is that battle casualties tend to be evenly spread across the elements of the group - so you're unlikely to have one cavalry element completely destroyed while an infantry element remains untouched, for example.