Obvious things to Freelance
Logo design, branding and any kind of "brochure" website.
Early mock-ups / screenshots of your product.
Pros: Cheap and fast.
Cons: There is little time for building the kind of collaborative relationship that a designer needs to have with the client to really understand your full vision. It's almost never that you get work from a freelancer that you'll be 100% in love with.
Very little accountability: Their job is to build what you tell them to.

Obvious on when to hire full time designers:
When your product or service is already successful enough that there is a long backlog of tweaks, new things to design, etc.

When the work you got from freelance designers was enough to get you your first customers, investors, or funding of some kind, but feel you now need to take your design to to the next level before launching:

Pros: Full-time resource who will almost always be less expensive than a contractor (in billable time) given the number of hours they will be working.

Someone who is accountable to deliver what you need and whose work can then be measured and quantified.

Cons: Ongoing financial liability.
Recruiting: Hiring *great* designers is really difficult. They are more sought-after than technical talent in most markets.