A few reasons I clean before they arrive:

• They're not paid great so I'd rather not inconvenience them too much and let them get into their next gig faster.
• I don't want them putting non dishwasher items in the dishwasher and potentially ruining them.
• You will lose items because they're organizing your place and will put stuff not where you expect it.
• They tend to do a deeper clean when they're not focusing on cleaning up the smaller stuff you can take care of yourself