Why classification of living things are needed?
It helps with organization and with grouping creatures in a way that allows us to see overarching trends, and disparities.
If you classify creatures together you expect similarity between them in some form. This can help with hypothesizing possible evolutionary changes over time. If you classify a creature with a group of fishes, you could hypothesize how changes over time resulted in that type of fish. If that same fish like creature was actually a sponge, you could then make deductions about how fish like features appeared in a separate group. We can easily changes as relations in common ancestry vs physiological adaptations.
We can also recognize trends across classified groups and use that to make hypothesis about new creatures of creatures that have gone extinct. We look at things the group has in common, or how creatures in the same group have diverged.
It also helps so that when discussing creatures internationally, we all have a standardized way to know what we are talking about.