Everytime you post on Instagram while using hashtags, your post shows up at the very top of the “Recent” hashtags list.
What this means is that each time you post, you get another chance to be seen by thousands and hundreds of thousands of people.
So the more you post, the more you show up in the top of the recent hashtag list. There are many large accounts with over 100k followers that post upwards of 6 to 8 times a day.
They understand that posting more means more exposure. More exposure can lead to more engagement and more followers.
When you post more, you also give yourself a better chance of showing up in front of different groups of users that use Instagram at different times.
For example, if you only post in the morning time, you may be missing out on all of the people that get on Instagram in the afternoons or in the evenings.