st.mk is mainly a zoning tool.
Like, “this is the area I can controll and now try to get in on me”.
Besides what Cipher said it’s also good at shutting down sonic booms and fireballs. Like if you stand right at it’s max range and throw it out at the proper timing you will see it stuff their attempt to throw a sonic boom or fireball. At this point guile or ryu usually has to do a st hp instead to try and crush counter the st mk, so you usually want to delay your st mk or try a pre-emptive st hp to stuff their hp attempt. It’s all rock paper scissor where you are trying to guess the right normal for the right time and spacing.