WEll, there are good agents, who are worth a lot, and others who aren't. A good person, one worth their weight in gold, can find you a gig where and when you want it. Eg you want two dates to fill in a tour in September. Between one date in Edinburgh and one in Penzance three days later. A good agent will get you a gig in Manchester, then a gig in Bristol. An average agent will get you a gig in Bristol, then a gig in Manchester.A bad agent won't get you anything(or it might be you are an unsaleable band!).
Basically, any agent can ring you up once a year and say"I've got you a Southern Comfort pub promotion gig in Scunthorpe in November". Worth knowing, perhaps, but not as much use as an agent who can get you a nice gig with a listening audience in Scunthorpe when it suits you to be there!
Personally, I would say good gigs are worth 20% of anyone's money. You just have to decide what it's worth to you.