How are droids related to the harvest?

Droids are used to manage the moisture vaporators on which the farm depends for its existence. The Lars family make a reasonable living from the harvesting of atmospheric moisture which they sell to other local sub-surface hydroponic farmers (like Camie's parents), to the nearby settlements like Mos Eisley and which they use to grow their own crops to eat and sell.

The need for human/droid labour is greatest when their and others crops are close to harvest because a) the need for water is at its greatest when the crops are closest to harvest which means that their vaporators need to be running at top efficiency to provide the much-needed water or risk suffering a catastrophic crop harvest failure and b) that the vaporators seem to particularly suffer from sand-clogging at harvest time due to the combination of continuous use, lubricants and (presumably) slightly-higher-than-average moisture in the air.

The Official Star Wars Fact File #8 gives us a little more info about the need for droids and humans to service the towers, clean filters and communicate with the vaporators to ensure that they remain in good working order.

MAINTENANCE: Exposed to the planet's frequent sandstorms, vaporators can easily become clogged by sand particles attracted to the machine's sticky lubricants. Maintenance droids ore commonly used to ease this unending routine.

and

AN UNCERTAIN LIFE: Maintaining the vaporators is a prime concern for farmers, many of whom employ farmhands or droids to monitor and maintain their vaporators continually, as loss of water production can often lead to bankruptcy in such a precarious enterprise. Using droids assists the smooth running of a moisture farm, particularly during the labour-intensive season prior to harvest. In addition to agricultural robots and maintenance droids, it is often necessary to utilize the more sophisticated robots needed to translate the binary programming language used by the vaporators.