Answer: The Gemara (Berachos 6b) writes that there are special praises and rewards for one who establishes a fixed place for their tefilla. Rabbeinu Yonah (Berachos 3b) explains that this refers to davening in the same shul, though if one has to daven at home, they should do so in a designated place. The Rosh (Berachos 1:7) and Shulchan Aruch (OC 9:19), however, disagree, writing that one must have a designated seat within their shul.
Some poskim follow both these rishonim. Hence, the Aruch Hashulchan (OC 90:23) writes that one should have both a designated spot in shul as well as at home if they sometimes daven there.
Nonetheless, the Magen Avraham (OC 90:34) and Mishna Berura (90:60) extend this to davening within four amos of one’s seat.
If one has an important reason to move, however, one may do so. Thus, the Kaf Hachaim (OC 90:118) writes that if one is being disturbed by others, they should move seats.
Accordingly, if one find’s another person in their seat, they should sit elsewhere rather than bother them, ideally within four amos of their regular seat.
In conclusion, it is important that one davens in the same shul regularly and one should try, where possible, to daven in the same seat, though not at another’s expense.