Marinades are a mixture of wet and dry ingredients that you soak raw foods in to flavor and tenderize. Acidic substances like vinegars and citrus juices make meats tender by permeating the meat and breakdown the connective tissues. These provide a flavor of their own and may be all you need to make a perfect steak. For instance a great way to make fajitas is to soak a flank steak in lime juice for a few hours (or overnight), grill and slice into strips. If the steak you have is a tough cut then you will want to marinate if for a few hours to make it tender and juicy.
Of course some steaks don't need to me tenderized. Good, tender cuts will only go soft in a marinade. You want to preserve as much of the original texture of a steak as you can. This doesn't mean you can't marinade a sirloin, but you don't need it to tenderize. Marinades based on oils will coat steaks and helping to keep them moist if they happen to be extra lean or extra thick (requiring longer cooking times), or if you have a preference for a well done steak. If you decide to use salt (and I suggest you try it) use a coarse or kosher salt. The coarser crystals last through the grilling better.
Steak not to marinate: Club, Filet Mignon, Kansas City Strip, Loin, New York Strip, Porterhouse, Rib, Shell, Sirloin, T-bone, and Tenderloin
Beef has the great property of taking well to most any spice. Seasonings can by used in marinades which can carry flavor deep into meat, they can also be rubbed onto the surface and they can be added to sauces poured over grilled steaks. There is a lot you can do to add flavor.
Of course people will say that a good steak doesn't need added flavor. I agree. If you have a good cut of meat, grilled to perfection then you don't need to add flavors. However, poorer cuts can use some sprucing up. Also if you want to achieve a specific dish like a Teriyaki Steak then you need to add seasonings before you cook.
Salt is a big controversy in the area of steaks. Professional chefs will tell you that salt keeps meats from browning and browning is an important aspect of the properly grilled steak. Some will tell you that salt will dry out a steak. Others will say that salt enhances flavors. However the same chefs that tell you not to use salt will marinate steaks in soy sauce. The trick here is that if you are going to use salt then you need to use it in moderation.