No, it's an obligation. You're required by law to do it, but it's not necessary, unless you feel the need to do it and your conscience would kill you if you didn't. A necessity is something you can't do without. An obligation is something you are made to do.

While some taxes should be paid (for instance i'm paying my property tax which pays for the roads around me to be maintained and lit, my garbage to be collected and disposed, things of that sort) some like the income tax shouldn't. They are nothing but a robbery.