For instance, let's say you have a company that's making an app. Your company is ten people. The venture capitalists will often throw money at your company to grow it to a larger size.
I can only speculate on why this is, but I would guess that it's easier for them to sell the company for a large price if the company itself is relatively large.
For instance, HPE paid $1.1 billion for Nimble Storage. They have 1100 employees. If Nimble only had 30 employees would they be able to justify that valuation? Probably not.