Tasks and Issues are different objects with different capabilities. They would / could cover two different kinds of work.
First, some basic definition.
Think of tasks as "planned work". Tasks frequently show up for us in project templates or as part of a larger project. They are linked together in some way (we're working on a set of tasks to bring a project to fruition).
Think of issues as "ad hoc work" -- i.e. unplanned work. e.g.1 Issues frequently crop up in the middle of a project. You probably weren't expecting that the designer can't get their work done because the custom form on a project didn't get filled out. e.g.2 To Teale's point above, teams rarely hire employees on demand, so you probably don't have a project with tasks every week for onboarding the new hire of the week. But... whenever it is that a new hire DOES get hired, someone can submit a request.
Second, let's consider complexity.
Tasks can be linked together. You can keep expenses on them. You kind of need a paid license to effectively set up tasks.
Issues are much much simpler. You definitely can't have sub-issues within an issue. On the other hand... anyone can submit an issue/request.
In many companies, the user breakdown is usually along the lines of 75% free licenses and 25% paid licenses. What are my 75% free license holders in the system doing? Usually, they're initiating a work request or reviewing proofs.
When my company is happy with their projects and tasks, this is when we look upstream and try and get the work into the system even sooner. This is where the request can really come into play as a way of marking a placeholder for future work to be assessed. Again, using Teale's example: HR wouldn't get into the system, create a project and assign tasks to IT to order and setup a laptop. IT will do that, as long as HR just tells them who and when.
You didn't ask, but the major down side of working in requests and tasks at the same time comes because many users have custom reports they like to refer to, and there aren't any reports that are able to combine reporting requests and tasks AND sort by due date.