I can't quite put my finger exactly on what it is, but it feels like there's a difference between the kind of assistance that a senior developer asks for and the kind that a junior developer requires.
I mgiht guess that a senior developer asks for assistance when it will save them time - they could work it out, but it'll be more efficient for everyone involved if they get some assistance - whereas a junior developer would be lost without it.
I think the difference is that a senior developer will ask for help when it is a multiplier on productivity, whereas a junior will ask/need help when it is a linear gain in productivity (ie the difference is knowing how useful that help will be).
To some extent, absolutely. A junior is more likely to not know where to start, or to get stuck on a wrong path. A senior is more likely to need someone to bounce architectural ideas off, or some pointers on how to get started with some new technology.
But ultimately, anyone can get stuck hunting for a typo causing an inexplicable problem. Sometimes you just need someone to explain your problem to, just to help your own head to solve it.
I mgiht guess that a senior developer asks for assistance when it will save them time - they could work it out, but it'll be more efficient for everyone involved if they get some assistance - whereas a junior developer would be lost without it.