There's nothing a church can usefully do that can't be done at least as well by a secular organisation.
However, even in secular nations, church provision of social services, both formal and informal, frequently pushes out secular provision, leaving religion with a monopoly. As this automatically denies or obstructs service to those of the wrong religion, or of none, it represents a lower standard of service than should be provided.
As a result, even the best intentioned and most valuable provision of social services by religious organisations is harmful to society.
And of course, many church activities are positively harmful and corrosive to society. The good stuff isn't the norm, and isn't as good as an equivalent secular activity would be.