If you think about it, a question about buck vs boost can be rephrased as a question about batteries. If you can choose between buck or boost, that must mean you have some control over the raw input voltage of your project, which usually means being able to put more or fewer batteries in series, or choose between AA/NiMH (~1.2V/cell) and Li-Ion (~3.7V/cell).
With that perspective in mind, one can see that the limitations of batteries can easily force you into one choice or the other.
As an example, if your project needs 5V and all you have is NiMH cells (which go from about 1.4-1.1V over a decent discharge), you can either:
a) power it with 1 to 3 batteries in cells and use a boost converter
b) power it with more than 5 batteries in cells and use a buck converter
(there are other options but let's not get lost in the details)
With option a), the fewer cells you use, the more current will be drawn from each cell (a lot more). If you project needs a lot of power, even 3 cells might not be able to handle it, in which case you'll be forced to either use option b), or add another string of cells in parallel (which comes with its own set of problems).
Even if the cells can take it, more current means more problems. The converter will be less efficient the wider the gap between the raw and output voltage. Cells become more inefficient the higher their discharge current. If the batteries are also not immediately adjacent to the converter (can happen with some robots), high current and low voltage means a significant voltage loss in the battery wires, which further decreases efficiency.
One significant advantage of option a), however, is that you can get away with using the minimal number of batteries if you don't need too much power. This can be critical in weight-sensitive applications like small robots.
Option a) has the advantage of using fewer batteries, which can be critical in a weight-sensitive application like a robot.
With option b), the current is reduced the more cells you place in series. This means less stress on each individual cell, better performance from the batteries, reduced losses, less heat. Overall this can add up to massive efficiency gains. I was once able to significantly (20-30%) boost the runtime of a battery-operated Raspberry Pi by switching from 4s2p NiMH with a boost converter, to 8s with a buck converter.
One major disadvantage of using a buck converter is that there are some designs out there have a deadly failure mode where the full raw input voltage will be present at the output. You'll want to provide some protection downstream from the converter to ensure your expensive circuit isn't destroyed if the converter malfunctions.