Most (all current?) welfare schemes have a disincentive for work because earning income disqualifies you from welfare, and getting a poor job can make you worse off even if the welfare amount is below the minimum wage, since going to that job often also does add extra expense (transportation, caretaker for kids and other dependents, etc).

With unconditional basic income, however, all the job income is extra, which is a big difference in motivation especially for people who can't hold a full-time job because of health issues or having to take care of others.

Furthermore, standard welfare systems don't handle structural unemployment well - they don't provide a long-term (decades, not months or years) means of support to able-bodied people that can't get a job. Historically, that wasn't much of a problem - able-bodied people could get a job if they really wanted most of the time; if there was an economic slump, then it went away in a couple years (and welfare could help ride it over), and if they couldn't get a job, then retraining was a solution. In the future, the situation is different, we should expect millions of people, eventually a majority of population, whose labor is economically unnecessary (it can be done by machines better if these people aren't allowed to participate) despite any reasonable retraining. This issue is(will be) new, and current methods clearly won't work.

People respond to "carrots and sticks", and UBI brings a much more sensible set of "carrots and sticks" than progressive taxation + classic welfare.