Every mid-sized company usually has legal counsel on staff (we have two lawyers for ~100 employees, but we might be a bit specific), that will escalate to an external law firm as necessary. Usually this would be when dealing with extraordinary events such as lawsuits or negotiation of a complex agreement.
Most companies generally have a marketing department, but will engage marketing agencies for major product launches.
Whenever an organization generates more than a few hundred invoices per month, it has an accounting department. But it isn't expected that this department would have expertise regarding international tax treaties.
Companies with significant cash flow have a CFO … and yet most companies go public with significant assistance from investment bankers.
Brick & mortar retailers have their own architects and interior designers, but you wouldn't expect them to draw the detailed plans of new building.
Every IT department has someone with networking skills … and expect that individual to know every possible architecture, network design and configuration option a vendor's equipment supports. This put-upon network professional usually toughs it out through help from the Great Oracle of Google, eventually getting some kind of Rube Goldberg concoction working.
There's something wrong with this picture. Work smart, not hard. Realize the limitations of your expertise and ask someone who specializes in the technology you have to deploy for help when appropriate. Your network will be grateful, although your vendor might not be.