Question

Why are electrophilic addition reactions the characteristic reactions of alkenes?

Answer 1

Here's an electrostatic potential map for ethylene,

Where red indicates high electron density.

In ionic reactions, what has the most electron density?

A nucleophile.

Hence, addition reactions of myriad of materials (an electrophilic addition occurs first in most mechanisms) about the planar double bond with different regioselectivity and stereospecificity are the most common reactions.

Here's a non-exhaustive list of a few ways we can leverage `pi` bonds to add substituents.