The last thing a seller wants when their home is under contract with a buyer is to have that sale fall apart due to a failed home inspection.
Some buyers use the home inspection contingency period to change their mind, at a high cost to the seller. The seller will most likely lose profit on the next sale, since new buyers sometimes think the sale fail was due to there being something wrong with the house.
If the seller has the home PRE-inspected before going into contract with a buyer, they will know exactly what condition their home is in. (There is almost always something that you didn't know needed fixing) Having the work done prior to putting your home on the market allows you to price your home higher than selling at an "as-is" price to make up for defects.
The pre-inspected home should sell more quickly and with fewer price or repair negotiations. The buyer can buy with more confidence, and the transaction will create much less stress for all parties.