If you listen to C3PO's complaints while walking on Tatooine, and read the novelisation of Star Wars, you'll realise that Tatooine is a very harsh environment for a droid, with sand infiltration damage, jawas stealing everything that isn't nailed down (and probably some things that are), and sand people just smashing stuff.
The implication is that droids simply do not last, and the Lars family had a suitable droid, but lost the use of it as the result of one or more of the aforementioned hazards, and from another conversation between Luke and Owen suggests that Luke wanted to go to the academy and couldn't because the family couldn't afford the droids it would take to replace him until just then with their most recent purchase.
Buying two droids may well have been a case of hedging bets against one of the droids suffering a mishap, an eminently reasonable decision, since within about a day, C3PO has had an accident resulting in the loss of an arm.
"This R2 unit has a bad motivator"