How could infrastructure such as utility lines, water and sewer pipes, highways, and rail lines be built or expanded if government didn't have the power to seize private property through eminent domain?
Builders would pay owners market price for property, instead of asking the government to condemn it, seize it, and hand it off to them as they so often do today. (See how some libertarian lawyers at the Institute for Justice are fighting it pro bono.)
Costs would be passed on to those who used the highways, etc., rather than asking a few people to bear it all.
Without eminent domain, many people fear that a few "hold-outs" could endanger important projects. In practice, builders would probably copy Disney's strategy for building its theme park in Orlando.
Disney didn't have the power to force people to sell, so it bought options on various properties through a holding company. If there were hold-outs in a particular area, Disney could shift its proposed boundaries with a minimum of loss.
In a society without eminent domain, developers would probably do the same. Individuals who wanted to keep their property, for sentimental or other reasons, wouldn't be forced to sell or take less than market price for their property. Everyone wins!