First of all,linguistics is the study of languages,and many of the teachers do not study languages,but teach languages.George Bernard Shaw once wrote:"He who can,does; he who cannot,teaches." Those who can study languages become linguists,those who cannot study languages become teachers of languages.
For some of the teachers who would like to teach languages well,they may need a little bit of theory,upon which they may be able to teach better.And most of the theories are,as it happens,discovered by linguists and summarised in linguistics.
Linguists,some of them,may also need to keep upgrading their theories,for which they also need to learn from classroom practices,which most of the teachers are so good at.Only by learning from the teachers can some of the theories by created sound.
If you are seriously interested in this topic,please read a book written by D.A.Wilkins,titled "Linguistics in Language Teaching",especially the first chapter,pp.1-37.