Speaking from a military science perspective they are simple concealment/light cover and dominate the landscape. You cannot pass one without being noticed and would therefore be reported upon and possibly attacked by a larger force.

Fighting from behind a fortified position was a good idea going all the way back to Roman encampment fortifications. A force of lesser armed individuals would have to take on the outer walls or breach a doorway to take out the possibly and likely smaller force housed within.

Blockhouses and watchtowers are usually easier to build and can be improved to become more sturdy earthen fortifications. Therefore the ability to withstand an attack of any magnitude would depend on the same factors that determine any conflict. Access to water and food, the condition and number of forces, the geographic area the fortification was in, the types of arms the enemy had at the time, etc.

I won’t speak directly to the purposes of colonial blockhouses and their use. I have a fundamental but limited knowledge of why they were employed. It should be noted that building a small and lightly defended position would happen because the area is in dispute like as in a frontier or colony. Larger fortifications would require more men, material, and maintenance. Blockhouses could be built using locally sourced materials and abandoned or destroyed after their purposes were finished.