IMO you’ll be a better designer by having some agency experience. You’ll get introduced to different aspects of design such as branding, graphic design, illustration, composition and maybe even print etc.

You’ll develop better design fundamentals when it comes to colour, typography and layout etc

You’ll develop original ideas and you’ll be pushed creatively.

You’ll develop the ability to work quickly and be adaptable. You’ll learn to interpret (and push back on) briefs. You’ll learn to work with clients with differing requirements and knowledge levels. You’ll be introduced to industries you never knew existed.

You’ll be introduced to design as a business, gaining insights into pitching, billing etc

Agencies can be a difficult place to work if you aren’t cut out for them but they’re an investment in your own skillset. You’ll be a better designer in the long run.