It seems counterintuitive that you would cut something in order to help it be longer. Let's dive into why it works.
As your hair grows, it gets longer. But daily life happens. We put our purse on our shoulder and sit it on top of our hair. We get our hair caught on things and rip it. We use hot tools and burn it. We use hair products that make it weak and the tension from brushing or styling breaks it. We burn it accidentally. We slack off on the hair treatments and the ends split and break.
There are hundreds of little things that break our hair off and the longer we go between haircuts, the weaker our hair strands get.
If we are getting regular 1/2 inch trims, we control how much hair comes off the bottom. If we aren't getting those trims often enough, we are gambling that we won't break off more than 1/2 inch in that time period and you know what they say about taking a gamble.
Split ends can rip all the way up the hair shaft before they break off, or anywhere in between. We have no control over where the hair breaks. Preventing the breakage is our best bet.
Everyone's hair is different, so talk to your stylist about how often you should be getting your hair trimmed when you're trying to grow it longer. As a general rule, chin length or shorter hair should be trimmed every 6-8 weeks until it reaches shoulder length. Shoulder length hair should be trimmed every 8-10 weeks until it reaches bra clasp length and anything bra clasp length or longer should be trimmed every 12 weeks. I never recommend going any longer than 3 months between hair trims.
Control the health of your hair, control the length of your hair!