When you say that someone knows their own mind, you are saying that they will not be persuaded to do things they don’t want to do. For example:
A parent discussing the safety of their daughter Susan, when she leaves home, might may say “you don’t have to worry about Susan she knows her own mind”.
Another example: a parent discussing that their child could be persuaded to do bad things might say “I know he’ll be ok because he knows his own mind”
The use of practical in your example is a second and separate descriptor.