A Basket is used to group a set of order lines. An order is placed for an individual item.
An Order can have multiple copies/units placed against it. A Basket can have multiple Orders.
It is highly advisable that you get in the habit of checking the cataloge first before placing an order for a new item. This saves unnecessary duplicate bibliographic records in your database and any need to merge/deduplicate records.
No, this is not necessary, you will still be able to mark items received without closing a basket. Closing a basket is good practice for keeping track of late orders which is discussed in this manual.