What part of speech is the word "be" and why?

Verb.

'Be' is a verb because it equates a noun to doing or being another noun or adjective.

'To be' is a central verb in English and is used not just as a lexical verb (on its own - 'I am human', where 'am' is a conjugation of 'to be'), but has auxiliary functions as well.

For example,

'You are going'

'You' is a pronoun. 'Are' is a conjugation of 'to be' and is the auxiliary or helping verb in this situation. 'Going' is in the gerund form, because it ends in '-ing', and is the lexical verb in this case, because it actually describes what is happening, while 'are' is merely grammatical.

Some languages don't have a word specifically for 'to be', and some philosophers regard it as a linguistic abomination, because it implies assumptions about existence that aren't necessarily true.