Actually, the expression more money supply is not clarifying at all which effect is desired, whence your (really natural) question.
What is implicit behind this expression is that commercial banks will have an easier access to (re-)financing. If commercial banks play their role of Money multiplier -- which is not that obvious since they may prefer to clean their balance sheets --, they will basically be more prompt to grant credits. Put differently and using an abusive but pedagogical term: they will be psychologically more prompt to waste money, and thus to stimulate demands here and there.
Since growth is very often conceived as being demands driven, more demands (of goods or whatever) at a constant level of supply (of those goods or whatever) in turn means inflation, until levels of supply, here and there, adjust and create jobs, generate dividends, etc...
To conclude and summarize
Basic Reason of Why the Money Supply Needs to Increase?
Because one needs commercial banks to be more relax and play their role.