Knowing the proper form or devising the right form to work specific portions of the muscle is key. Working off of feel can mislead you to think your working a specific muscle while your not. Obviously you should feel the target muscle working when doing an exercise aimed at it but that doesn't mean your building it to full potential.
For example, If you are going by feel on lets say a incline press and you can feel it working your upper pecs you may be mistaken that they getting worked more then your lower pecs when in reality you may feel your upper pecs working and they make get 80% of the stimulation but only 20% of the growth because your lower pec has more muscle fibers and is more conducive to growth even though it receives only 20% of the stimulation it obtains 80% of the growth spurred by the exercise.
In such a case it is better to go with a guillotine press where the bar is lowered to where the clavicle and sternum meet while flaring elbows as far back as possible. If available, it should be done on a smith machine allowing the user to grip the bar as wide as possible with a false grip and have the hands at an angle. All of this form makes sure the parts of the pec attached to the clavicle are very isolated through the bottom range of the press. This allows you to develop your upper pec to a greater potential then if you stuck with incline BB presses because you could "feel" it working them
Another example would be someone who can feel the flat BB bench working their pecs but in the end the form of this dictates that the shoulders and triceps do most of the work. I'm not saying you can't build a large chest with the bench alone and some people have but for most the accepted form for this exercise hits the shoulders and triceps with much more stimulation then the chest regardless of techniques such as splitting the bar or using wider grips elbows flared.
Isolating a muscle entirely is an impossible task but by selecting the proper form that targets the portion of the muscle where you want to gain instead of going on where you feel things working is a major reason I feel many physiques are out of proportion end up having the effect of a unmade bed. No one cares if you are showcasing your deluxe king size Tempurpedic with massage and heat if it doesn't look appealing to the eye. If the goal is bodybuilding and selecting the right exercise form to "add clay in the right places" as arnold would say is the name of the game.
Most people won't even try this because after they realize they have to cut the weight from their regular bench in half or more their egos will get in the way when in reality they would look so much better for it. Size without shape is absolutely grotesque