Why do I have to pay a retainer to your law firm right after the fee quote/legal service agreement is signed?

This is a general practice oflaw firms because, when carrying out legal work for clients, we have to arrange the right human resources needed for the job as well asincur some necessary expenses for services purchased from outside suppliers related to your assigned work and so we need to receive some money from you for us to cover costs incurred. The retainer will be deducted from the service fee of the last invoice issued to you, and if there is still a positive balance of the retainer, it will be refunded.