Most players are aware that a verbal declaration of any action is binding( in turn).
BUT many are not aware that after a verbal declaration, the chips must still be placed in to the pot.
Verbal declaration is not "enough".
Placement of the chips in to the pot is required in order to prevent any confusion that might occur due to mis-understanding or mis-statement of intentions.
For instance: a player might say "i wonder if i should call", where an opponent or dealer might only hear "i call", hands might then be shown prematurely and an argument as to whether what was said will ensue.
If the player pushed the chips in, there is not a question as to the intent.