Typically, as soon as your lender receives the accepted purchase agreement, they immediately begin preparing to finalize the loan so that the closing can take place as scheduled. The appraisal is their first step in this process. Since appraisals involve an additional third party, they order this as soon as possible.
If you prefer to possibly delay closing, then my advice is to submit the purchase agreement to your lender after you have completed the home inspection. This could easily add an additional week to the closing process but it can prevent an additional cost if the inspection fails.