This line means that love is not a necessity of life. It builds the theme that love is not needed by comparing it to not being needed to fuel the body. The sonnet expresses this repeatedly throughout the poem. This is one of the examples of how they did it. The author describes how love is not needed to fuel the body like food or drinks are. Love is not a necessity of life.
This line builds the theme and is important. That fact is because it is saying that loves will not save your life or provide you with something you need to live. It is saying that it will not be the float to save you from drowning in life. It will not save your life if you are struggling. It will not float in comparison to how men will sink. It is basically saying that it will not help you it will just be there.
Nor yet a floating spar to men that sink
A lot of contrast is shown in this poem. The poem repetitively compares love and survival situations to say how neither of them will allow the individual to make it out alive. Love will not keep you alive anymore than the useless or nonexistent objects described in the poem would. Love is not food or drink that will cure you of dehydration or starvation. Love will not help you float when you are drowning. Love will not help you in any meaningful way according to the poem.
The turn seen in line 7 refocuses the poem. The beginning of the described how useless love was and now it is talking about how willing people are to make friends with something that can be so painful yet so unhelpful.
Yet many a man is making friends with death
I might be driven to sell your love for peace,
This line takes place after the turn. Before the turn, the poem was talking about how useless love was. However, it now is talking about how it can be traded. It is starting to talk about what they would rather have then love. For example. they would trade someone's love for them for peace. To them, love is useless and they would rather have useful things like peace in their life even though they cannot literally trade someone's love like that.
Just like the other lines in the poem after the turn, this line is talking about how love is pointless to life and that there are better things. That is what the theme of the poem is and this line describes how they would rather trade the memory of a date for food to survive off of later.