Nothing for an engineer is more satisfying than seeing your code be used to solve real problems for customers. Positive (and negative) feedback from people actually using the product you built is the lifeblood of those who really care about making a difference.
Conversely, there is nothing more 'soul-sucking' than to realize that something you have devoted months or years of work, effort, and thought into is thrown in the dustbin because there was just no business support for it. This can happen when your company is bought out by a competitor just so they could kill your project. This can happen because a SVP decided on a whim to suddenly change the whole direction of the company. It can happen because someone in the management chain just doesn't have a clue what they are doing.
Conversely, there is nothing more 'soul-sucking' than to realize that something you have devoted months or years of work, effort, and thought into is thrown in the dustbin because there was just no business support for it. This can happen when your company is bought out by a competitor just so they could kill your project. This can happen because a SVP decided on a whim to suddenly change the whole direction of the company. It can happen because someone in the management chain just doesn't have a clue what they are doing.