1. It's the original pilates
Joseph Pilates called his method Contrology, and it started on the floor. His 1945 book, Return to Life Through Contrology, teaches 34 exercises, every one of them on the mat. The reformer and the other machines came to support that work, not to replace it.
2. It can be harder than the reformer
On the reformer, springs can share the load. On the mat there's nothing to share it with: you lift, lower and hold your own body weight, and your core has to control every inch of it. That's why strong people are often surprised by how much a mat class asks of them.
3. The Hundred is a breathing drill
The most famous exercise in pilates is named after a count: 100 small arm beats, breathing in for five and out for five, ten times. The burn in your abs is real, but the point is the breath: learning to keep breathing steadily while your center stays switched on.
4. Your core is more than your abs
Pilates calls it the powerhouse: the deep abdominals, the muscles along your spine, your hips, glutes and pelvic floor, all working together with your breath. Most mat exercises train them as one unit, which is why posture is often the first thing people notice changing.
5. Fewer reps, more control
Mat classes rarely chase high numbers. Most exercises are repeated a handful of times, slowly and precisely, because control is the training. Five perfect reps teach your body more than twenty rushed ones, and leave nowhere to hide.
The short version
No machine and nowhere to hide: that's exactly why mat pilates works.