It was very clear why Owen decided to buy C-3PO:

OWEN: What I really need is a droid who understands the binary language of moisture evaporators.

But they appeared to be really interested in acquiring an astromech droid since when the first one (R5-D4) blew up, they decided to get R2-D2.

What use would they have for an astromech droid?

It was very clear why Owen decided to buy C-3PO:

OWEN: What I really need is a droid who understands the binary language of moisture vaporators.

But they appeared to be really interested in acquiring an astromech droid since when the first one (R5-D4) blew up because of a bad motivator, they decided to get R2-D2.

What use would they have for an astromech droid?

It was very clear why Owen decided to buy C-3PO:

OWEN: What I really need is a droid who understands the binary language of moisture evaporators.

But they appeared to be really interested in acquiring an R2 unit since when the first one blew up, they decided to get R2-D2.

What use would they have for an R2 unit?

It was very clear why Owen decided to buy C-3PO:

OWEN: What I really need is a droid who understands the binary language of moisture evaporators.

But they appeared to be really interested in acquiring an astromech droid since when the first one (R5-D4) blew up, they decided to get R2-D2.

What use would they have for an astromech droid?