They could be your future clients.
|If you've already worked with him, he should trust you again... or maybe not?|
Many former employees at Google have left the big company to start their own luck... And be later on acquired back by Google for a large lump of cash.
If you're starting a business, make sure you keep good relationships with former colleagues too, you never know what future entails. Plus they'll be happy to see what you've become!