她到碼頭搬東西 (tā dào mǎ tóu bān dōng xī/ She was at the port carrying cargo) means she did the action of 搬東西 at the place of 碼頭.
她搬東西到碼頭 (tā bān dōng xī dào mǎ tóu/ She carried the cargo to the port) means she 搬東西 to 碼頭.
The VP 搬東西 (bān dōng xī/ moving stuffs) requires a PP as a goal argument. We can’t say 我搬東西 (wǒ bān dōng xī) without any location or goal. It has to take a PP which can be either pre-verbal (means location) or post-verbal (means goal).
