because you want to know how much money you got, when, and from whom, and be able to reconcile that with the money you see in your bank account. the detail is that money transfers themselves aren't instantaneous, aren't paid out instantaneously, and aren't always singular - ie you get paid by the company, but also pay taxes on that, so there are three parties to that one transaction. for a single person, the overhead isn't super justified unless you're a total nerd about it (nothing wrong with that!), but if there's an accounting department with an bunch of peopke, they can more easily keep track of issued invoices, payments due and payments recieved, for example.

it's not than all that's impossible with single entry, it's just way messier, so the overhead of double entry is considered better because the books can be more fully audited.

if you're a freelancer, you may get paid net 30 or 60 or even 90. but you still need to know how much money you have right now, how much money you're owed; financial health, P&Ls etc.