It can increase performance a little bit and allows you to hide the items if you don't want them.
Press space to search for everything (can be added to the tooltip).
Doesn't affect the bookmarks.

Same option in the Sodium/Embeddium Extras mod:

/src/main/java/toni/sodiumextras/EmbyConfig.java#L249-L251

hideJREI = BUILDER
  .comment("Toggles JREI item rendering until searching", "Increases performance a little bit and cleans your screen when you don't want to use it")
  .define("hideJREI", false);

/src/main/resources/assets/sodiumextras/lang/en_us.json#L107-L108

"sodium.extras.options.jei.title": "Hide JEI Until Searching",
"sodium.extras.options.jei.desc": "If enabled, JEI item list will be hidden unless you search for something. Press space to search for everything."