org-table-auto-blank-field

org-table-auto-blank-field is a variable defined in `org-table.el'.
Its value is

t

Documentation:
Non-nil means automatically blank table field when starting to type into it.
This only happens when typing immediately after a field motion
command (TAB, S-TAB or RET).
Only relevant when `org-enable-table-editor' is equal to `optimized'.

You can customize this variable.