The interface of balsamiq has two main functions:
1 - Stop you from fiddling
You won't spend time doing unnecessary things. If there's no option for color or rounded corners, you won't worry which shade or radius to use.
2 - Communicate that it's a sketch.
Of these menu mockups, which is more likely to receive comments about the M not being the right font for the logo? The top one makes it very clear that this is not a stylistic mockup but a functional one. So all the feedback from the client would be about positioning.
Using an obviously sketchy style keeps all involved parties more focused on the layout and interactions rather than styling and details.
Of course this has the tradeoff of potentially ending up with issues down the line. For example, I used yellow on red for the logo, but the inverse for the buttons. The black and white sketch doesn't show any difference, so you wouldn't get feedback on that if you use balsamiq.
In the end, both routes are valid options, and it depends on the circumstances. If you're specifically working on wireframing for complex sites or interactions, you may want to work with balsamiq. However if it's a smaller project and only 1 dev/team you might want to integrate the sketching, interaction, and first visual mockup stages by using something like Photoshop or Sketch.