Professional Development budgets can be hard to spend. Spluring everything on a conference gets you some half-decent benefits - something to put on your self-assessment for your review cycle; a couple of days out-of-the-office; some half-decent food, snacks, and corporate gifts; and finally the chance to travel to an interesting new city on your company's money.
How you put going to a conference on your self assessment?
> Personal improvements this year
> Went to a conference, learned a bunch of stuff on company time
> Business value added this year
> Went to a conference and got better!
Or what?
Some employers require all employees to complete a certain amount of professional development every year. You want your patent lawyers to know about the latest developments in patent law, your HR people to know about the latest developments in employment law, and so on. In this case, merely having accomplished the professional development goals you set last year counts as goals being achieved.
Don't like that? Maybe your employer is starting a big project that's going to produce business benefits but relies on a new technology. For example, if you were previously an expert in virtual machines, and you've expanded your knowledge to Docker containers so you can help on an important new project - that's an achievement even if you were paid while you trained.
Perhaps you learned something at the conference you were able to put into effect directly, making your product better. That's an achievement - you're keeping the company at the cutting edge of technology!
Perhaps your employer is facing challenges with Foo, and the world's greatest expert on Foo was speaking at the conference, and you were able to quiz them about your challenges over coffee and get some great advice.
Perhaps after the conference you gave presentations to your peers at the company, passing on your new knowledge. That's helped the company by improving everyone's skills, not just yours - as well as demonstrating cross-team impact.
Perhaps at the conference you talked to some people who were interested in joining the company, and the chance to talk to an engineer in person along with the proof you're a cutting edge company following the very latest in technology has lead to more and higher quality job applicants.