Much ado about political appointees

WHAT exactly is the purpose of not appointing politicians to government-linked companies (GLCs) and government-linked investment companies (GLICs)?

The general assumption is that the heads/directors of GLCs should be technocrats or experts in the respective fields, as that would ensure better management of the companies. This assumption is more of an overkill because some politicians may have the expertise to manage these companies well whereas some may not.