I would tend to look at 是⋯⋯的 as a set for emphasis. With it removed, the sentence still means the same: 這一個要給李芳。
'For', a preposition, is not a terribly literal translation of 給, a verb meaning 'to give'. We translate 這一個(要)給李芳 naturally to 'This is for Li Fang' because it makes more sense this way in English, not necessarily because they are grammatically equivalent. However, since you're probing into grammar, there is the need to think more precisely.
The gift (這一個) cannot 'give' (給) itself to Li Fang. A doer is needed, say 我 'I', but it is understood to be the speaker themself and there is no need to say it explicitly. In fact, the sentence should be like this:
這一個//我(要)給李芳 Regarding this (gift), I am giving it to Li Fang.
這一個//我(要)給李芳 is a topic-comment sentence. 這一個 is the topic, and 我(要)給李芳 is the sentence.
要 indicates an intention to act and can be placed before the verb. The gift is not sent yet; the speaker has the intention to give it (要給) to Li Fang. There is ambiguity in 這一個我給李芳 in that the listener does not know whether the gift is sent or not.