Not many people know this, but there were actually two James Tobins.
The first James Tobin (pdf) said that there cannot be an excess supply of money, because if anyone did have an excess supply of money he would immediately run to the bank to get rid of it.
The second James Tobin said that people hold positive average stocks of money for the same reason anyone holds positive average stocks of any inventory: because it is too costly to keep running back and forth to the bank to get rid of inventory immediately and then get it back the moment before you need it again.
I might add that the bank is only one of a million places an individual could get rid of an excess supply of money. Any shop will take the stuff. Because it's money. An excess supply of money must mean an excess demand for something, but that something could be anything, and not necessarily anything the bank has for sale. But if you get rid of it anywhere other than at a bank, it just means someone else gets it. And if it keeps on circulating around the shops, rather than going back to the bank, that will change things like nominal income, and changing things like nominal income might also change people's minds about whether they really did want to get rid of it after all.
It all becomes a lot clearer when you realise there were two James Tobins, and they didn't agree. I think the second James Tobin was right.
(I sort of stole this from Laidler, but he's not responsible, of course.)