I always have the blood tests done on my senior dogs. I also have it done on newly adopted dogs. When I took Penny Lane to her first vet visit, they found low protein levels. As a result they were able to dig a little deeper and found a large ovarian cyst. My vet charges $80 for the extended tests. This is from a hand out my vet gave me:

Level I Baseline Screening
Beginning at a "human relative age" of 40 years, this basic screening uses blood tests
to evaluate internal organ function, especially the liver and kidneys, and monitors changes in red and white blood cells. It provides a "snapshot" of your pet’s health to help us monitor changes in the years to come.

Level II Senior Profile
Beginning at a "human relative age" of 45-50 years, the Level II profile is more thorough than the Level I Screen. In addition to the Level I blood tests, we check your pet for hypertension and glaucoma. A urinalysis is included, which aids in detecting early stages of kidney disease, one of the more common problems seen in older dogs.

Since dogs can't really tell us where they hurt or if they don't feel good - the blood tests can be an early warning system.