There are clear benefits to working with the same translation agency through time.
Firstly, when you build a relationship you get to know each other and learn how each party works. The more the agency knows about you, the more it will be able to help you. Likewise, the more you know about your agency, the more you know about what to expect and how to design your projects.
Secondly, the quality of the work you receive will remain consistently high if you work with the same provider as getting feedback project after project will allow the agency to build up terminological resources, to record your preferences, to book the translator you like the best for your projects or adapt its procedures to make yours simple and easy to manage.
Thirdly, as you work with the same providers you start building a relationship based on trust, which translates into many benefits for both parties.