Learn Portuguese (Portugal): Esta mochila é muito prática.

Esta means "this" and refers to something physically close to the speaker. If the backpack were closer to the person you are talking to, you would use essa (that). Both words are feminine to match mochila.

Why do we use é instead of está for "is"?

We use é (from the verb ser) for permanent qualities or characteristics. Being practical is a built-in feature of the backpack, not a temporary state like being open (está aberta).

Since mochila is feminine, why isn't it muita prática?

When muito means "very" and describes an adjective like prática, it is an adverb and never changes its ending. It only changes to muita when it means "a lot of" and describes a feminine noun, like muita água.

Why does prática end in an -a here?

Adjectives must match the gender of the noun they describe. Because mochila is a feminine noun, the base masculine adjective prático changes to the feminine form prática.