A GE Energy salesperson reinforced this idea for me. Suppose you are making a complex "systems"/"solutions" sale. To pull it off (get it thoroughly implemented—the basis for repeat business) you need help—LOTS OF—from a host of folks inside GE-wherever. These folks are congenitally overworked—and have a queue of salespersons needing help. Your Goal #1: Get an UNFAIR SHARE (this is the way the GE guy put it) of these insiders' time & energy & attention. The time of the sale is way, way too late. These are Internal Relationships you should have been forming and minding long, long ago.
The "simple" point: By developing a scintillating (extensive, deep) internal network you probably increase your external success dramatically.
(I call all this—inside or out—ROIR. Return On Investment in Relationships.)
Hint/Duh II: It takes a lot of time!