The tactile experience of a desk phone has many benefits. You can transfer/hold/merge calls without looking, even building up muscle memory for it. The speakerphones are often louder then a cell phone would be - and if you need some privacy, the phone is big enough to hold against your shoulder should you need to type.

You can also see who else is on a call if the system is set up for it. (Feature or bug? The jury is out...)

It's also nice at a big organization when they have tons of their own phone numbers and you can just dial 4 or 5 numbers within the company.