What a beat plan is actually promising
A beat plan is a commitment to a set of outlets: someone will appear on this day, at roughly this time, at this frequency. That predictability is worth a lot in trade. A retailer who knows your representative arrives on Tuesday saves questions for Tuesday, plans stock around it, and is less likely to fill shelf space with a competitor who happens to be present.
Which is why drift is expensive. When visits become irregular, the outlet stops planning around you, and the relationship becomes transactional. Nothing about that is visible in a monthly sales figure until it has been happening for a quarter.