Use the checker when you know you want to increase rent, but you still need to know whether the figure is supportable and which Section 13 pack fits the risk.
Before you serve Form 4A, you need to know whether the proposed rent is a sensible figure to stand behind. The checker gives you that first read before you commit to the paperwork.
That matters because a Section 13 file can look tidy on the surface while still being weak underneath. A clean form does not fix a poor figure, stale evidence, or a story that changes once the tenant pushes back.
The useful result answers three questions quickly: where the rent sits against the local market, how strong the evidence looks, and what you should do next.
If the evidence is strong but the proposed figure sits high, the issue is not weak preparation. The issue may be that the figure is too ambitious for the current market calculation.
Use the Standard Section 13 route when the figure looks supportable and you want the notice, rent summary, and service record aligned. Use Defence when the rent may still be arguable but the tenant is likely to test it.
That is the point of the free tool: it turns research into a practical next step instead of leaving you with a pile of guidance and no clear route.