Hello L K. In the dental profession, we call the retainer thing a "flipper". I am presuming you are referring to the same thing. It is entirely up to you whether or not you choose to get this for your daughter, however, the importance of it is to retain space needed for her permanent teeth to grow into. Having space between teeth allows the remaining teeth to eventually move over, in whatever direction there is a space, and can create problems later in life. Remember, her "permanent" teeth that will come in later in life are going to be much bigger than her "baby" teeth, so there needs to be adequate room for those to grow into, and not be pushed around. Sure, she could go without the flipper, and have her teeth shift, but later in life she'll probably need braces to correct the overlapping of the teeth. In most cases where braces are needed, tooth removal is also a part of the process to promote growth where the teeth can fit into the space/s they need to fit into. This is just something to think about :)
BTW.... my daughter also had to wear a flipper for a while. Her top two teeth had to be removed because she broke them in half vertically - one while biting a Lego, and one because she hit her face on the wall after bouncing on the bed. Two huge no-no's!! lol Her teeth are beautiful now, but then again, she wasn't predisposed to needing braces to begin with.