SAN FRANCISCO — Perhaps nothing illustrates the clubby atmosphere of Silicon Valley quite like the tangled web of directors and consultants at many of the area’s companies.

The many concurrent roles that Valley players often take on — executive at one company, director at several others and at times consultants to still more — is a big reason the Valley is such a good incubator for new businesses. A startup software developer can greatly benefit from a director who has spent years at larger rivals.