Growth plan accounts get a monthly request limit, and most businesses never come close to it. For the ones that do, particularly multi location businesses or anyone running a seasonal spike, add ons fill the gap without forcing an upgrade to a different plan.
When the base limit stops being enough
The clearest sign is simple: requests start failing or queuing because the monthly allowance has run out before the month has. This tends to show up around busy seasons, a landscaping business in spring, a cleaning service after the holidays, or simply as a business grows past the volume it was sending when it first signed up.
One time vs recurring add ons
A one time top up makes sense for a short term spike, a busy month that is not representative of the rest of the year. It adds a fixed block of requests that does not renew, so there is nothing to remember to cancel afterwards. A recurring add on makes more sense once the higher volume looks like the new normal rather than a temporary bump, since it renews automatically each billing period at the same rate.
How to decide which one fits
The simplest test: if the same spike is likely to repeat next month, a recurring add on saves the trouble of buying a top up by hand every time. If it was a one off event, a single purchase covers it without committing to anything ongoing. Either way, requests added through an add on stack directly on top of the base monthly limit, so existing usage and billing are not affected by the choice.