Software agencies: pros and cons?

Hi there! I would love to hear about your experiences and thoughts on working with software agencies. When do you think it is reasonable to hire them, and why? What concerns or negative experiences have you had? I'll be honest: I work for a software engineering agency, and I'm looking to better understand my target audience, your perceptions of software engineering agencies, and any concerns you might have. I promise I won't direct message anyone unless you specifically ask me to!