One of my common tasks as a freelancer is invoicing. I've used FileMaker Pro for this task for more than a decade. I host a custom built database on my Mac Mini which is shared and accessed on the local network via FileMaker Go on the iPad. FileMaker Go is an excellent app that lets me do almost anything the full version of FileMaker Pro does. It does not allow for editing the design of the database itself, which is to say, creating layouts, fields, etc. But I have full access to all data and can edit and add new records as I need.
Each client gets a record and then multiple invoices can be created for that client. Each invoice can have an unlimited number of line items added. When I'm ready to send the invoice I click a button which brings up a dialog and with one click to confirm I get a new email with the invoice attached as a pdf. I can then add any note into the body of the email and send. Simple and effective and done right from the iPad. The invoice get's marked as having been sent and dated. With a glance I can look at my invoice list and see those that haven't been paid as well as the outstanding balance for all invoices. If I need to find invoices in a certain time period it's easy as FileMaker Go has great search features. All of this from the iPad.
The only thing I need to use the Mac for is editing the design of the database which I do only rarely. But it's true that a Mac or Windows machine is necessary to add new fields, scripts, layouts or anything related to the building of a database.