The individual "tmp" directory within each user's home directory will be recreated if it is missing the next time log processing occurs for that account.
The individual user tmp directories are primarily used for web statistics, including the statistics configuration file "stats.conf" that can be used to disable/enable specific statistics programs. There may be other usage depending on the circumstances, such as in cPanel 11.25 -- if Roundcube was converted from MySQL to SQLite by the server administrator then there may be the user's Roundcube error log and or a temporary backup of Roundcube data saved from the time of conversion.
The individual user tmp directories are primarily used for web statistics, including the statistics configuration file "stats.conf" that can be used to disable/enable specific statistics programs. There may be other usage depending on the circumstances, such as in cPanel 11.25 -- if Roundcube was converted from MySQL to SQLite by the server administrator then there may be the user's Roundcube error log and or a temporary backup of Roundcube data saved from the time of conversion.