There are many academic studies on this exact topic. Two theories are given weight: 1) people feel a sense of obligation because they received something (the natural human tendency to repay debts) 2) being able to buy votes is a forward-looking signal that shows the party/candidate is serious and may be able to provide further benefits down the line.

Further, political operators can at least guarantee that they went to the polls. This way, they can influence turnout.

See https://www.cambridge.org/core/journals/world-politics/artic...