One of the tasks a gardener must do yearly, or more often, is clean up the garden. In the ornamental garden you’ll have old dead perennials that must be cut down and dealt with to make room for the new season’s growth.
In the past I’ve used a lawnmower for this, it can be hard getting it up into the bed, and it is an indiscriminate shredder, and it does go low to the ground sometimes impacting green foliage, but in the fall, before the snow blankets everything, it is a decent tool. It also finely chops up everything into mulch onto the bed itself.
In Spring I do not like it as much, simply because some things might already be growing. Also, I’m not sure it is good for the lawnmower itself to be attacking somewhat woody perennials.