Homeschoolers take classes online, they go to local classes and co-ops, and they take lots of educational field trips. For some subjects however, it is best to have an instructor to come to you.
You will need an in home instructor when your child desires to learn a subject that is beyond your grasp or to develop a talent you don't have.
A piano instructor that comes to you will save you dealing with traffic, and allow you to squeeze piano lessons in on a busy day. A math tutor that comes to you will give your child one-on-one attention that is not possible in a classroom or even in a small group.
To choose an in home homeschool instructor, it is best to put out a call to other homeschoolers seeking word of mouth referrals. You want someone who has shown effectiveness in teaching, and who you will feel safe about them coming to your home. You can try out someone who has never taught a homeschooler before, but you should be sure to interview them thoroughly and check references.