As a conference organizer, I can attest to the value of a tech conference if there are educational/tech sessions ad guests that are useful to you. Ie, ones you'll learn some useful skill or get ideas for further research.
Many big conferences turn into parties where the sessions are generic/dull and it's a sales-pitch for either the conference organizers or the vendors. I don't need to attend another session about "Why XYZ is awesome". I want to know how it's awesome and see demos of what it is capable of so that it can blow my mind.
At a good conference:
Attendees can learn useful skills or get worthwhile ideas from tech sessions, learn about new technologies, industry trends, etc...
Sponsors gain viewership & notice by everyone involved.
Vendors can get their products in front of a crowd of people from their industry.
Speakers improve their brand and public speaking skills.
Networking (an often ugly word) consists of meeting and talking to people in roles like your own and learning how they solve problems you may also encounter.
Your employer stands to gain from sending you to the conference in that you'll learn/network, which makes you a better employee, and will probably have some fun, making you a happier employee. The worst thing for them is to maintain a fleet of employees that stagnate and don't learn new skills.