If what you do is paste in some text and get one result back (a summary, a rewrite, a draft), a well written prompt is the right tool and you're not missing out. An agent is for work that takes several steps and needs to use tools or make decisions along the way.
Examples where an agent earns its keep:
- Reading every new support email, looking up the customer in your records, drafting a reply and flagging the urgent ones.
- Researching a list of companies: find each website, pull out what they sell, score the fit and write the results into a spreadsheet.
- A weekly report that gathers numbers from a few places, checks them, writes the summary and sends it.
A good test: if you'd normally do it as a checklist of steps that involve opening other apps, it's an agent candidate. If it's one question in and one answer out, keep prompting. It's also smart to get your prompts reliable first, because an agent is basically a chain of prompts and it inherits their weaknesses.