If you don't have access to landlines then the GSM Desk Phone provides you with a phone that will work anywhere within the coverage of your chosen mobile network. If you do have a landline, the GSM Desk Phone provides an immediate backup if the landline fails.
The GSM Desk Phone is more permanent, reliable and capable of working in fringe areas than a "normal" mobile phone. It is also less likely to be mislaid, stolen or damaged.