Question

Why is the term marginal important in utility analysis

Answer 1

Because 'marginal' implies that the satifaction (utility) from consuming an additional good continuously gets smaller and smaller. Whereas utility primarily looks at the overall satisfaction gained

Answer 2

The utility gained from consuming the last unit of a good.

For example, if you consume one cake, your marginal utility may be quite high – say 10 units of satisfaction

If you consume a second cake (in same day) your marginal utility of the second cake is likely to fall quite a bit to say +2
If you consume three cakes, you may even start to feel ill and get negative utility from a third cake.