SCRIPTS is where the uncompiled version of a level's scripts should be stored. Import it just after the BEHAVIOR lump and you will be providing a way for other script-savvy folks to see the specifics of the work you've done and perhaps gain ideas from your code usage. Providing a SCRIPTS lump is a courtesy only, not a necessity for ZDoom to function; however it is inconsiderate to make some mega-cool project and then not provide the scripts for it so the rest of the community can benefit as well, unless of course it is just a demo that you have released and you feel you must preserve your secrets until the day of the big release.
The SCRIPTS lump is also used by editors based on Doom Builder such as Ultimate Doom Builder to store and edit a levels' uncompiled ACS scripts using their script editor, therefore it is neccesary to keep the SCRIPTS lump to be able to continue editing your levels' scripts with those editors.
Note that the lump containing your uncompiled scripts does not have to be called SCRIPTS, as this is only the naming convention used by Zeth, WadAuthor and Doom Builder. DeePsea, for instance, can store uncompiled scripts in a lump named SCRIPTS or SCRIPT##, where ## is the map number (so scripts for map01 are SCRIPTS01 etc etc). A scripts lump is not vital to running a wad and thus can be placed anywhere in the WAD (although placing it under the BEHAVIOR lump it corresponds to will make it immediately clear to what level it belongs) and may have any (non-reserved) name.
See ACS for more detail about scripting.