I'm intending to replace outer tie rods on a 2000 ford explorer, and they will need grease added with a grease gun. Are there other parts of the car that should be regularly greased that justify purchasing a grease gun?
I depends on the specific car. But in some cases there are multiple grease fittings that should be greased periodically. In addition to your tie rod ends there are upper and lower ball joints, wheel bearings, u-joints, driveshaft slip joint, to name a few.
In recent years auto manufacturers have started leaving out the grease fittings and calling the parts "lifetime lubricated" but that really means they are lubricated for the lifetime of the part. In some cases you can drill/tap a hole and insert a grease fitting yourself to prolong the life of some of these parts.
In my opinion a grease gun is an important tool in every DIY toolbox. In addition to autos my grease gun find use greasing trailer axles, riding lawn mower, and other things that I find need to be lubricated.