In a socialist economy (in theory!) the lives of people have intrinsic value regardless of their productivity. In a (purely) capitalist economy, a person's value comes instead from the value they contribute to society. Under socialism, work is a necessity to keep welfare solvent. Under capitalism, a worker's welfare is the market price of their work.
Under (ideal) socialism, automation reduces how much human labor is required to provide welfare. Under capitalism, automation displaces workers from jobs, creating structural unemployment. It's up to the workers to justify their continued existence by re-skilling and finding a new job.
A major problem with socialism was laziness. If income isn't tied to work, why work at all? And if nobody works, there's no income. Usually, this is answered by authoritarianism - you must justify your existence to the Party, rather than the market. This is less relevant with automation, since machines don't need financial incentives. Socialism should become increasingly viable as automation makes UBI cheaper.
...whereas the endpoint of capitalism, it seems to me, is serfdom, as it becomes increasingly difficult to justify any wages, so a person's value becomes purely a function of the capital they own - such as shares in increasingly autonomous megacorps.