Instagram is important for all types of businesses, not just photography. It might not be my main means for finding clients, but it sure boosts credibility. If I searched for a certain company's instagram account to find they only had a couple dozen followers, it might turn me away. There is no excuse for lack of followers or lack of an account with these simple steps:

Vulnerability. Keep you account public. Having to request to follow someone is one more step in someone's busy day.
Post regularly. Set a reminder on your phone or in your planner to post once a day or several times a week.
#Hashtags. The more hashtags you use, the more users will see your photo, like it, click on your profile, and follow you. I strive to add about 5-10 hashtags per post.
Have a theme. And stick to it. Keep that theme consistent with your brand. No one wants to see a random cluster of food photos, selfies, sunsets, and cats. I save that for my personal account ;)
FOLLOW. This is the most effective step. Follow as many users as you can. Like and comment on photos that interest you. You are guaranteed to get courtesy follow-backs from this tactic. If said account doesn't follow you back within a couple days, you can always unfollow them. I like to search hashtags related to my industry (#photographer, #naturephotographer #canon6D, etc), and then follow those users who's interests and businesses are similar to mine.

These steps have me growing at a rate of 150 followers/month. If you stay consistent that could mean 1,800/year! You might not get tons of clients compared to other outlets, but growing all of your social media accounts is an important way for your brand to look up-to-speed.