Do we have to buy all of our computers through our MSP?

A provider that requires every purchase to run through them is worth questioning: it usually means opaque markup, and it is a mild form of lock-in. But the opposite extreme causes real damage too. The consumer laptop from a big-box sale arrives with the Home edition of Windows (which cannot join business security and management systems), minimal memory, a one-year consumer warranty, and a lifespan measured in frustration. It costs more in support hours and early replacement than it saved at the register. Hardware standards exist for support reasons, not sales reasons.

The arrangement that serves you best is freedom of vendor with adherence to spec, and it belongs in the agreement: you may buy from anyone, and purchases follow the MSP's minimum recommendations. In practice that means business-class machines, a Pro operating system, current security hardware, and a real warranty. The MSP provides the specification or a quote; you are free to buy from them, from a distributor, or from the manufacturer directly, whoever prices it best that week.

That said, many businesses end up buying through their MSP voluntarily, because the convenience is real: the provider handles procurement, sets the machine up completely before it ever reaches the employee, and deals with warranty claims so you never sit on hold. That is a fine outcome when it is a choice backed by transparent, itemized quotes, and a problem when it is a mandate. Related question worth reading: whether an MSP will force you to replace hardware that still works.