Watch the Chords I-V7 online guitar lesson by Ton van Bergeijk from Licks to Riffs Guidebook: Blues

When we go from the root to the dominant, let's say from C to G7, we notice that the only note in common is the G. The other notes from G7 - B, D, and F - are pretty near the other notes of the C chord (C and E). You'll have to change more notes if you want to use the same lick over G7 as you do in C. Also, the move to the dominant is at the end of your chord scheme. To avoid monotony, these differences are quite welcome in that part of the song.