I can think of two benefits to an adjustable desk:

1. 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.
2. 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.