ND110 is not a must.
Nothing is a must.
ND comes in a lot of flavors and a lot of brands. Use the correct ND (number of stops) for correct desired effects.
more popular ND filters:
1 stop - Hoya ND2, B+W ND101, Cokin P ND2, Lee ND0.3.
2 stops - Hoya ND4, B+W ND102, Cokin P ND4, Lee ND0.6.
3 stops - Hoya ND8, B+W ND103, Cokin P ND8, Lee ND0.9.
6 stops - B+W ND106
9 stops - Hoya ND400 (8.5 stops actual)
10 stops - B+W ND110 (around 9 stops actual), Lee Big Stopper.