Apple MacBook Pro RAM
It's like upgrade your house to have another room. Wether or not it justify's its cost depends on wether you use it or not. In general, I'd say 4GB is the minimum to go by these days, I'd opt for 8GB for most non-casual users.

Considering how expensive a MacBook is, I'd suggest going the extra mile to ensure your purchase lasts you well into the future. If your model is not a retina model (I.e. the RAM is user serviceable), I'd strongly recommend a 2x8GB kit like this one. At $130, it's only $30 more expensive than the 8GB Apple offers, but is DOUBLE the memory. (it's faster too, but your MacBook will bottleneck the speed difference, so it won't actually count for anything)