You may have heard of the term "scope creep" in management:
"Scope creep (also called requirement creep, or kitchen sink syndrome) in project management refers to changes, continuous or uncontrolled growth in a project’s scope, at any point after the project begins. This can occur when the scope of a project is not properly defined, documented, or controlled."
(From Wikipedia "Scope Creep";  Lewis, James (2002). Fundamentals of Project Management (Second ed.). AMACOM. pp. 29, 63. ISBN 0-8144-7132-3.)
This "creeping" can also happen in roles too.
When a role "is not properly defined, documented, or controlled," role creep is bound to happen.
One example is the role of supervisor or manager. In some unfortunate cases, some supervisors role creep / micromanage due to the fear of "letting go" of the responsibilities that should be handled by their direct reports. Have you experienced your manager stepping on your toes all the time? The failure to delegate is something that I have struggled to tackle, and I have not found a solution for it (it probably needs to be dealt with on a case by case basis anyways).
If you join an organization that is attempting to expand rapidly, your role will likely experience role creep. You'll probably be asked to wear say 3 hats at the time of hire, but when an organization is forced to "do more with less," one day you'll notice that you have 20 hats instead of the original 3. Some people find this enjoyable but others will find this exhausting.
When the organization is large and stable, role creep may be the key to get you a raise or to get you promoted. If you voluntarily wear more hats and prove to the employer that you can take more things on (or new things on), that may open doors for future opportunities. Let me tell you my relationship with role creep: Whenever I wanted to wear a new hat and my supervisor said "no" to it because it was out of my "scope"... that hat was - consistently - the hat I ended up wearing in my next job. So if you're faced with a situation where you want to wear a hat but get shut down, it may still be worth doing research around that hat (even if you can't wear it immediately). You never know when you'll get a chance to wear it. It could be right around the corner...
I'm Candice and I doodle with the intensity of the doomguy.