I can think of two benefits to an adjustable desk:
- Better chairs at a lower cost. Most office chairs (and chairs in general) are designed for table-height desks, so you'll find a greater variety of multi-point-adjustable ergonomic options that'll improve your posture while seated. From a corporate perspective, these chairs are also more versatile when the office changes size or layout because they'll work just as well around the conference table as in the cubicle.
- This one's purely a business reason, but also the main reason an office manager will have on their mind: the employee they hire to replace you might be a different height. Cynical, I know, but an adjustable height desk means they can accommodate anyone they hire now or in the future, and they've got to justify office expenses on a multi-year timescale
For you, an existing employee who already has a desk and chair you like, the adjustable desk will probably be a downgrade. For the office, it's a smart business decision that also means comfier chairs for everyone.