[SeanAppleby]

If you want to move a large amount of money into an asset, you either have to place bids and wait for people to take your order at a particular price, or you have to pay a slight premium to dig into the ask side of the book and pay more money as you consume orders and liquidity.

If you place an ask and the price moves down it might not fill, so you have to move it over time while the price slips.

People provide liquidity by seeing your new sell order and filling it quickly, or by leaving a large number sell orders that people can take immediately, which results in money moving more quickly and consistently.