Because you can then rent for a bigger amount (conditional on it being the tenant that needs what you have fitted), which is not so hard
It also works in reverse though - when you leave a premises, you don't have to strip out your custom fit-out and put the space back into how it looked before you left. The landlord can't say "I never liked your choice of lighting, so put it back the way it was", for example.
Though, yeah, depending on the area I don't expect adherence (or enforcement) to be much effective, which doesn't seem to be the case for where this restaurant was located though