When you mention marshmallow, most people think of the pink and white fluffy sweet things. And not the delicate wild flower above, Althea officinalis. Of course, the sweets or an ancient version of them, used to be made with extract of the roots or leaves of the plant, although modern marshmallows, you may be relieved to hear, no longer contain any trace of the plant. Now it's just sugar and glue (not really). 
Marshmallow contains a large proportion of what herbalists call "mucilage", which when mixed with water forms a kind of gel which is used topically to reduce skin inflammation, or can be taken internally to calm inflammation of the throat or stomach. "Mucilage"sounds pretty unappetising, so I guess a large amount of sugar would have helped to make it more attractive to the consumer/patient. I've never tried making anything with my marshmallows, but I do like the plants, they are tall, with rather soft velvety foliage and delicate flowers of the palest marshmallow pink - I didn't plant them, they just appeared on their own in the bog garden (so weeds then really) next to the pond. I have an overgrown area of yellow irises and the marshmallow grows happily amongst them, together with a few Willow Herbs and Purple Loosestrife, which I know people think are dreadful weeds but I quite like them and I justify them as being a good dinner for the Elephant Hawk Moth,and bees and butterflies generally. The marshmallows look a bit like a wild and more delicate form of hollyhock, Althea rosea, to which they are closely related
All kinds of mallow make good cottage garden plants, - especially the white form of the common mallow Malva moschata
which again appears uninvited and is a sparkling white low growing flower that goes on for ages, so I usually leave it to grow. It's easy enough to pull out if it gets a bit too rampant. And then there's the good old hollyhock, Althea rosea, which I love but it always gets rust in my garden so I don't usually have them unless my brother gives me a few of his spares.
And finally there's the shrubby mallow, or Lavatera
which I grow as an easy fast growing plant for difficult places. It's unfussy and never fails to produce its mass of bright pink flowers in the summer months. This one is a proper garden plant and numerous named varieties are available, notably Lavatera Barnsley which is a softer colour than this ordinary one in my garden, but they all need to be cut back after flowering though, or they just get very woody and bare at the base. They strike easily from cuttings, and it's just as well to have a few coming along as the plants are often short lived and die away after a few years.
