The imprest system is a method of controlling petty cash: a cashier is given a fixed float (the imprest amount) and reimbursed at the end of each period for exactly what was spent, restoring the float to its original level.
Its main advantage is that it provides a convenient, controlled way of meeting small, routine items of expenditure, such as stationery, postage, or minor travel costs, without going through the full cheque or bank payment process for every tiny transaction, while still keeping records tight because reimbursement is only made against vouchers for actual spending.
The imprest system has nothing to do with making high profits, since it is a cash-control mechanism rather than a source of income; it is not designed to reward the person holding the float, since any cash held is business money, not personal remuneration; and it does not, by itself, make the preparation of final accounts easier, since petty cash is only one small part of the overall accounting records.
When a question asks about the purpose or advantage of the imprest system, think "control over small cash expenses," since that is the concept being tested.