Its because God told us so!
D&C 89 is the full revelation Joseph Smith recieved from our Heavenly Father after asking if they should use Tobacco. Verse 4 kinda talks about why we have the Word of Wisdom
"4 Behold, verily, thus saith the Lord unto you: In consequence of evils and designs which do and will exist in the hearts of conspiring men in the last days, I have warned you, and forewarn you, by giving unto you this word of wisdom by revelation—"
Its a warning! God knows what can and will happen to us. He knows everything! Should we take such a warning lightly? Of course not!
There's 5 simple things that the Word of Wisdom says to stay away from:
Coffee, Tea, Tobacco, Alcohol, and Illegal Drugs
I know I know, it seems pretty hard. But with this simple commandment comes promised blessings! Verses 18-21 say:
"18 And all saints who remember to keep and do these sayings, walking in obedience to the commandments, shall receive health in their navel and marrow to their bones;