why don't you or could you

[Darlene]

when you make a request, do the following sentences differ in any way?

1. Why don't you help your mother set the table?
2. Could you help your mother set the table?

a lot of thanks in advance

[RonBee]

when you make a request, do the following sentences differ in any way?

1. Why don't you help your mother set the table?
2. Could you help your mother set the table?

a lot of thanks in advance

The two sentences differ a great deal. Depending context and tone, the first sentence is either a simple question or it is a critical remark. The second one is more likely to be a polite request.