There is no necessity to buy steem power as per Steemit Blog Logic. you can grow if you will get upvote for your blog from users specially big reputation user. But it's not easy.

if you want to grow fast, you can invest into steem power as per your capacity. you need the help of your reputation level, voting power, need the help of voting bot. It will help you temporarily to grow fast.

you need to write good quality blog regularly for long term.