When I copy a node's snapshot to its /var/lib/cassandra/data///, and run 'nodetool refresh', what will happen to this newly replaced sstables and original sstables, because the original sstables is still there, and some new writes still in commitlog and memtables.
Does backup one node's snapshot make sense? Because other nodes may have the the data with more recent timestamp.
Does backup one node's data make sense?
Not in a multi node environment. If you're removing a node from the cluster or running a rolling upgrade maybe, but not for backup purposes on a live cluster.
This is a problem that tends to get addressed by using a parallel ssh tool. The example given by DataStax is pssh. This will create all the snapshots at the same time on each node giving you consistent data assuming you're not dealing with heavy writes (since C* is eventually consistent).
