Why do established public schools hire non-established substitute teachers?

In fact, the school is not willing to hire "substitute teachers" at all, so doing so is a helpless move. There are many female teachers in primary and secondary schools, and it is common for them to take long leaves for childbirth. However, front-line teachers are all in trouble, and taking leave will directly affect normal teaching. Although schools have to report to their superiors every year to request more teachers to alleviate teacher tension, the funding for primary and secondary school teachers is a burden on the local finance, and approval is difficult and difficult. In order to ensure the normal progress of teaching work, the school has no choice but to temporarily hire "substitute teachers".