Email solutions are company-wide, and companies are big.
Tons of companies use exchange on-prem either for historical reasons or legal reasons (e.g. they can't store email in the cloud - yes, this is very much a thing).
Beyond a certain size, companies inevitably have to support teams on different OSes. You have to let the accounting and finance guys use Windows (they'll die without Excel), you have to let the creatives use OSX or they'll throw a fit, and you have to let some in house IT teams use Linux.
But they all have to use the same email platform, and if it's on-prem exchange, the Linux guys are in a tough spot.
Tons of companies use exchange on-prem either for historical reasons or legal reasons (e.g. they can't store email in the cloud - yes, this is very much a thing).
Beyond a certain size, companies inevitably have to support teams on different OSes. You have to let the accounting and finance guys use Windows (they'll die without Excel), you have to let the creatives use OSX or they'll throw a fit, and you have to let some in house IT teams use Linux.
But they all have to use the same email platform, and if it's on-prem exchange, the Linux guys are in a tough spot.