In what sort of way “v$transaction” view will help DBA's to improve the performance of the query?

Question

I've just started learning some DBA tips and techniques in Oracle and I came across v$transaction view. I've read some articles regarding this view but some things are still ambiguous ! I want to know:

1. On which circumstances do DBA's normally use this view?
2. What does USED_UREC column shows us? the description "the number of undo records held by an active transaction" is a bit confusing .
3. In what sort of way this view will help DBA's to improve the performance of the query?

I was wondering if you could help me understand the question above. Thanks in advance