I feel like the best thing to do is write about the personal experiences that have shaped you to become passionate in the field, whether its a cousin who's had speech therapy, a class you found interesting, research you did in your undergrad or a volunteering experience that brought you to the career; you can't go wrong if it is something that you personally identify with. I think that they look for that connection.
Another great tip is to have it read over by multiple people, so they can let you know what resonated with them, what parts confused them, etc. so that you can get your final draft clear, concise and impactful.
I personally wrote about the classes I took, some of my volunteering experiences, and my work as a SLP-A, which has all brought me to solidifying my desire to become an SLP.
Please reach out if you have any other questions! Good luck with apps!!