So you're going to bathe your dog. Why waste your time brushing him first? He's just going to get wet.
It turns out that grooming your dog prior to a bath is actually very useful and important. Removing excess dirt, debris, and dead hair prior to the bath prevents exposure to contaminants to areas of the body, such as eyes and genitals, from runoff bath water that can contain bacteria. Also, matted or tangled hair that gets wet becomes an irredeemable mess that cannot be worked out and will need to be cut out with scissors. While fur is dry there is a chance you can work out a snarl or two before bathing! Also, grooming tasks such as nail trimming and excess hair removal can be performed prior to bathing when your dog is still fresh and less likely to object because he has lost patience with the process!
Grooming before a bath can be relaxing for your dog, if he likes a good brushing. If your dog is stressed by grooming, then you may want to break up pre-bath grooming and bathing into two stages, with a break in between to let your dog recover. What works best varies for each individual dog, so know your pup and whether he needs the process broken up or is just a diva that loves an extended spa day!
It may sound counterintuitive to groom your dog before you bathe him, after all, he is just going to get wet anyway! Grooming prior to a bath actually is very productive as it removes excess dirt and hair that can just tangle up during the bathing process. Make sure to take breaks, especially if you have a long-haired dog that will need a long time to brush out and bathe, so that you both don't get tired and impatient with bath day.