The Advanced panel does take up more space on your screen but brings quite a lot of capabilities with it.
It is designed for more 'power' features that you may use occasionally or all the time if you are a busy store.
Because it takes up more screen real estate you tend to need a bigger screen if you are using it regularly.
Here is Search mode with it on and hidden: (respectively)
and the same in Create mode:
BRM will remember where you have the setting - shown or hidden (and store it for your user ID)
Here are the extra functions the advanced Panel brings;
Detail Search (detail search) - by customer name, customer ID, item ID, notes, group name etc
Date Range control (when) - set the ranges of your reservation searches
Client Ticker Panel (who) - bring in clients from a variety of sources, e.g. Tablets
Date control (when) - preset for which dates 'create reservation' will work