Wheel bearings last a long time, but they do wear out, just like clutch discs and brake pads. Your Fiat 124 Spider has a very conventional setup for front wheel bearings that make changing them a not-too-difficult proposition. If you have never replaced wheel bearings before, or it has been a while, watch the video, read through the procedure in your Workshop Manual and set aside the parts, tools and enough time to do a good job.
If your bearings are truly worn out or damaged, replacing them will make the car drive noticeably better. Vibration and noises that they may have caused will disappear. The car will track better down the road and, of course, it will roll better too. That can show up as improved performance and better gas mileage.
Starting at the wheel-side of the assembly there is a grease cup cap at the hub center. Then a 27mm nut, a washer specially made to fit the slot cut in the top of the wheel spindle, the outer bearings, the outer bearing race, the inner bearing race, the inner bearings and finally, at the innermost part of the spindle, will be the inner grease seal.
All of the parts inside the hub, like the bearings and races, will ride in a thick coating of axle grease. Be sure to "pack" the bearings well so that all of the force of the wheel turning is put against the layer of grease and not against any metal-to-metal contact. When you take the assembly apart, be sure to clean everything completely. This lets you inspect the parts for wear or damage and makes a smooth reassembly no problem at all.
When you set the bearings at the end, by rotating the wheel and tightening and loosening the center nut, notice how the wheel tightens up and then turns smoothly as you move the wrench back and forth. You are actually drawing the hub assembly tightly together and making it snug against the spindle. Too loose and the wheel will vibrate and roll erratically. Too tight, and the car will not roll well and the bearings will wear prematurely.
Always wear eye protection when using a hammer. Protective gloves are a good way to stay clean and you may want to wear a dust mask, as the brake calipers and hub can have lots of brake dust on them. You don't want to inhale any of that!
Follow your shop manual and take care of your Fiat 124 Spider. It’s Way Better Than Walking!