system
Given the following post, respond as if you're commenting on Reddit:
human
[Doctor Strange] Why did Doctor Strange use the time stone AFTER he entered the Dark Dimension?

Stephen knows he is up against something... otherworldly, to put it lightly. The Dark Dimension is Dormammu's home turf, so the entity is going to sense if someone comes inside. 

So, then why did he decide to use the "checkpoint" function of the time stone, once he had entered the Dark Dimension? Why didn't he set it just before he entered through the dimensional tear? Dormammu could have killed Strange as soon as he landed on that surface to set the time stone; why risk it?
gpt
Because it's not a 'checkpoint' for Strange, it's a jail cell for Dormammu.  

If the time loop isn't entirely in the Dark Dimension, it gives Dormammu an out that he can exploit to break it.  It is also unclear whether Dormammu would actually know, or care for that matter, if a human sorcerer entered his dimension just then.  He obviously doesn't fear Strange, so there's little risk of an immediate reaction.