You get services from someone and you pay them for it. You have a mutual understanding you once agreed in word and it worked out wonderful. You get what you need and your provider gets paid for it. No harm done. 

Well, for one the tax officials may argue – they see you made a payment and you have an invoice maybe, but if the invoice is not detailed, what are the services you receive? The agreement would be the place they are described. The tax officials may say that the expense is not related to business and make you pay tax or charge you with a fine.

Another good reason to sign an agreement is having general conditions of the service and payments being written and agreed. In this way both parties would exactly know what they’re liable and entitled to.

And if you at one point decide you don’t need the service? Could the other party say that they want to give it to you and you’re liable for paying for the service nonetheless?

All of this should be written and agreed within an agreement.