The Smith machine shoulder press trains all three heads of the deltoid with the front delts doing most of the work, while the fixed bar path takes balance out of the equation. That stability makes it a strong option for pushing heavy overhead volume safely, especially near failure.
The Smith shoulder press runs the overhead press along a fixed vertical track, deleting the balance demand so every bit of effort goes into pushing. That makes it the overhead press that is easiest to take to true failure safely, especially with the safety catches set, and a favourite for delt-focused hypertrophy work. Program it as the heavy press on shoulder days, 3 to 4 sets of 6 to 12 reps, or after a free-weight press as the drop-the-stabilisers second movement.
Set the bench or seat so the bar path passes just in front of the face, grip slightly wider than shoulders, and lower to chin or clavicle level before pressing to lockout. Because the machine owns the bar path, the setup position is everything: an inch too far back and the shoulders grind. Expect to press a little more than your barbell numbers, progressing in 2.5 kg steps, and always set the catches at the bottom of the range when training alone, since pressing to failure is the whole reason to pick this machine.
For delt size, essentially yes, and it is easier to push to failure. For total shoulder stability and carryover to free lifts, the barbell and dumbbells keep the edge, so most lifters run both.
Seated with a back support allows the most weight and the strictest delt work. Standing adds trunk demand but on a fixed track it buys little; seated is the default here.
The fixed path is forcing a line your joints disagree with. Shift the seat a few centimetres forward or back and shorten the depth slightly until the press is pain-free.
Log your smith shoulder press sets in 1RM.fit and watch your estimated 1RM update automatically as you get stronger.