What would the benefits be??

I drive a 99 silverado 74K. Im not too knowledgeable about engines...just the basics. Im noticing a decrease in power since I bought the truck 2 years ago so Im starting on tuning-up with the basics..just changed fuel filter today,changing plugs and wires tomorrow... but I was wondering What would be the benefits of getting a DIAGNOSTICS TEST?? Im clueless as to what this does. My warranty is almost up and Im fixin to bring it in so I can burn them for some more money..with little cost to me :) Im pretty sure Im having the same problem described in the front brakes recall issue..they slip when braking over bumps and railroad tracks. Anyways if anyone wants to help an eager learner...Fill me in!

now this is just my thoughts...good to change the fuel filter but i would also change the air filter. I would also check the oxygen sensors, just checked my repair manuel there are 4 O2 sensors. Depending on your engine knowledge you can do an O2 check yourself. If you have a repair manual it will give you a step by step on how to check O2 sensors. Haynes repair manuals are the best in my opinion. Hope this helps.