The idea behind keeping what you typed is that you are giving your short name to the food you are looking for and have found.
Many foods have pretty long names, and when you search, you are using the words that are natural for you, they are effectively how you name the food. With this approach, we save a lot of screen space (in mobile apps and on the web site). Also, short names are easier to scan and read.
We typically recommend giving "definitive" food names (i.e. searching/typing sometimes an extra word or two, to make the resulting food name unique for you). For example, if you always eat the same kind of hot dogs, typing "hot dog" is sufficient, but if you eat different kinds of hot dogs, e.g. beef franks and pork, such as "Oscar mayer wieners beef franks hotdog wiener frankfurter hot dog" and "Frankfurter pork hot dog frank wiene", you might want to type "beef frank hot dog" and "pork hot dog" respectively, although "hot dog" will be sufficient in each case. Again, in our opinion, this is easier on the eyes and fits nicely into the available screen real estate.
Deleting a Short Name
If you don't like or don't need a short name for some food, you can open Manage My Foods screen, then locate the food you need, tap on it, and from the appeared menu select Remove From Search History.