On the mechanical level they define character progress, on a narrative one – they help you flesh-out details about your gamedec’s past. When navigating the profession tree – you can as usually, choose whether you are a focused expert, brilliant in one chose field or rather a jack of all trades, master of none. The latter gives you specific freedom of approach when dealing with day-to-day challenges of medium difficulty. Still, it might be impossible to tackle complicated situations – you won't be in control anymore, left with a need to improvise and hope for the best.
For example, let's say you have a pack of three bottom tier professions, like Influencer, Medic, and Technician. That would allow you to work your natural charisma to manipulate people around, find hidden data caches that belong to virtualia designers, and give someone a CPR. But if you were to invest fully in the News-related branch (that is: go from Influencer to Journalist, Reporter and finally a Swapper), you would become a real information-broker. You have a press pass, own informants, ability to install and disable micro-cameras, and an inhuman ability to find your way into any database.
On the other hand – being an expert in a specific path lets you thrive in some environments but leaves you somewhat awkward in others. Not every case can be solved only by your vast knowledge of Chi-Tong - a programming language based on ideograms, allowing you to hack the hell out of everything, sometimes you might need to be able to perform a field operation or use the strength of your muscles.
Aspects, while describing your character further, are here for a different reason. They are used as a currency for unlocking different branches of the Profession tree. And Professions are – next to the Deduction system – your primary way of leading your investigations. Think of them as skills you obtain to unlock specific dialogue options or environmental actions. Want to manipulate your suspect? Professions are your answer – unlock a profession to be master in people skills. Locked door? Bypass the lock if you chose to go with a "Hacker" profession.