Why would you glue down a tire onto the rim?

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OK I have a pretty expensive mountain bike that I was going to sell as I mostly commute everyday since moving and don't really have the desire to cycle more on my days off!

Anyhow took it to the LBS and said service it as I will sell it. Mechanic said he did not have a new tubless tire for the back so would run a maxxis highroller with a tube in on to Bontrager race lite wheel. I said no probs the bike is going to sell anway. Well the bike did not sell so I'm in the process of changing the off road tires for greax evolutions to use the bike as a back up commuter bike. I come to take the back tire off and discover its been glued down on to the rim on one side by the mechanic?

Why do this? How the F*ck do I get the tire off without trashing the rim?