Why do we traditionally perform pressing/pushing exercises before extension exercises when weight training? There are a few reasons, with some less obvious (but no less important).
Muscle Group vs Specific Muscles
Press exercises focus on an entire muscle group. They also happen to be compound movements, meaning that there’s movement in more than one joint area (and parts of the limbs — arms for the upper body and legs for the lower body) while performing both the eccentric & concentric portions of the exercise. And because multiple muscles within that group are being worked, you can typically complete a relatively heavier lift (than extension-based exercises, that is).
Conversely, specific muscles (although sometimes more than one) are worked when performing a set of an extension exercise… which encompasses movement in one joint area (and one part of your limbs) only. There’s obviously some leeway for including a bit of movement in another joint area throughout the exercise… but it’s usually only to assist with completing it (as in the case of the amount of resistance being overbearing, because, once again, more joints equals a heavier lift).
With the above-mentioned taken into consideration, it’s better to begin your strength training workouts with a press exercise because:
It warms up all of the joints associated with the body part (i.e., entire muscle group) you’re currently exercising, simultaneously (whilst putting less stress on each joint when compared to extension-centric movements [and especially so if the latter were to be performed first]). In other words, it gets you ready for all exercises that follow.
Since you could push out more weight per rep (at least in regards to the first couple of reps) with a compound movement such as a press or leg squat, and that tends to be scarier (and more in line with basic pyramid-style training [albeit from a macro level, in this case]), it’s beneficial (and more effective psychologically) to perform those exercises first. This concept is, of course, founded on functional strength/powerlifting training methods — but it inevitably extends (no pun intended) to traditional bodybuilding (i.e., aesthetically shaping the body through both isotonic ranges of motion and isometric/static training), given the results (or type of results, rather) do cross over as well.
Similar to all associated joints being warmed up, a press exercise pumps blood into each & every muscle fiber within the (general/overall) muscle group being worked. This sets you up perfectly for an extension exercise, allowing you to (further) refine (and pump up) a specific muscle, regardless of whether you eventually alternate/circle back to a compound movement.