Permanent types can be combined, most notably in the form of artifact creatures (representing inorganic creatures and mechanical constructs) and enchantment creatures (representing inherently-magical creatures and spirits)
There are very few cards that do that
This makes Glissa, Herald of Perfection far stronger than it looks, beyond the scope of just the March of the Machine set
You can have a more Creature-focused build, utilizing the various Final Fantasy Creatures included in her precon, Scions and Spellcraft
Valued at a mere $0.30 upon Aetherdrift's release on February 14, the card has since spiked by 733% in 10 days, and copies now sell for $2.50 each, while foils go for as much as $8
All they do is reduce the toughness of a given creature permanently unless the counters are removed somehow