What are my cards good for once they have rotated out of Standard?
Based on how your question is worded, I assume you are talking about standard.

I believe you're really asking a more general question, "What are my cards good for once they have rotated out of Standard?"


They can still be played in alternate formats, such as Modern, Legacy, and Commander (and casual, as you've noted)
Some cards may be reprinted in standard again in the future, in which case you can use your old printings instead of buying new ones
From an investment standpoint, some cards may appreciate in value


I personally hang onto all of my old cards as my collection hasn't gotten too unwieldly in size yet. Whether or not your want to sell yours off is really just a personal decision about whether you think you'll be interested in different formats in the future.

If you don't want them anymore, you might also consider trading them for more current cards with players who do play other formats.