Question about Idle in TX

I know that Idle in Transactions are a problem, however I'm trying to
assess how much of a problem.

for example: If a java program connects to the DB and does "begin;"
and then internally does a "sleep 6 days"

Does that cauz any issues other than eating a connection to the database?

(note, nothing i have does this, i'm just trying to understand)

I know that "Idle in TXs" can interfere with Vaccums for example, but
I'm not sure if that's due to them usually having some form of lock on a
table.

Thanks,

Dave