you can use without -dir ofcourse. meaning will be same i.e., bu ne? what is this?//bu nedir? what is this? -dir is other kind of -imek verb. if -imek or -dir is not used, anyway it is estimated as used in the word. i.e. bu kim? who is this? bu bir ogrenci in the answer, ogrenci is verb. so it is actually "bu bir ogrencidir". but as you see, -dir is not used. look this: bu bir kalem bu bir kalem idir (present tens) i canceled and it is used as kalemdir, instead of kalem idir. past tense shape is idi. ex: bu bir kalem idi, bu bir kalemdi. same thing. -idi, -dir etc. makes for you easy to understand which word is the verb in sentence

-dır -dir = probability

You should mostly omit them, esp in casual talk (ordinary speech), but the use of these suffixes adds emphasis in particular contexts and help us express a truth. e.g. X topluma büyük katkıları olan önemli bir kişidir. We wouldn't use -dir for unimportant matters like "Masadır" "Gözdür". Then just "masa", "göz" is enough.

-dir in english used as am/is/are. to reply as student or to reply i am a student is the same meaning when you are asked