There is no downside really, the box works normally as the built in software isn't modified in any way.
Firstly if your Foxsat isn't connected to your home network then there is zero advantage as well.
Roughly how it works.
The custom firmware sets up a website on your box that you log into with any common webbrowser using pc's tablet pcs smart phones etc. Once installed through a very simple interface you install applications known as packages on the box.
Examples of the extra capability you get via computer using packages are.
A user customisable epg, you get to use a proper keyboard for searches.
The capability to stream SD recordings to other networked kit without affecting the main box user.
Streaming the live output a little behind real time by streaming the time shift buffer file. (works for HD).
Direct access to the HDD hard disc either in Windows Explorer or using ftp.
A fully functioning remote control with favourites icons and recordable button macros.
The capability to view and set manual recordings for non-freesat channrls without having to switch modes. For instance you can have a channel number for Sky news that simply works by entering the number.
Capability to control the box over the internet. This includes setting recording reservations. Say you are going away for longer than a week and need to set a reservation for a new series not yet in the epg. No problem do it remotely anywhere there's a net connection.
When combined with AV2HDR the capability to upload your own video content to the internal HDD.
Probably forgot a few.