In short: Some parts of evaluation a small team can genuinely do alone and some parts shouldn't be done alone, and knowing which is which protects both the budget and the credibility of the result.
Not every program needs an outside evaluator. These pages cover what you can evaluate yourself, the tools worth using, simple survey design, and knowing when to ask for help.
The choice between an outside evaluator and doing it internally turns on independence, capacity, and what the funder requires, and sometimes a full evaluation is more than you need.
Work-based learning holds people when the mentoring layer is built on purpose, and whether it's working can be checked while it runs.