A couple of reasons, Fred...
In fact, I strongly recommend a y-valve instead of only a tee or wye fitting. One reason--the macerator isn't barrier to water rising in the overboard discharge line to fill the tank. A y-valve that's kept in the pumpout direction becomes a backup for the seacock (that too many people leave open, even though they shouldn't). It also allows the macerator to be installed very near the tank discharge instead of above the tank when the tank discharge is below the water line. Why is that good? Because the further from the tank and higher it is, the longer it takes to prime...and the longer a macerator takes to prime, the more wear and tear on the impeller from friction heat. And finally, marine sanitation laws in coastal waters require that a system be secured from overboard discharge while inside the "3 mile limit." Without a y-valve, the only way to secure the tank against illegal dumping is to secure the thru-hull--by wire tying it or removing the handle. If you have a y-valve, which is often much easier to reach than the seacock, all that's necessary is to secure it.
