Relation between rate of adsorption and the critical temperature of a gas
You can argue that the effect of the solid is to "seed" formation of the condensed phase, as a set of interactions that lower the chemical potential below that of the gas, leading to "condensation" (well, adsorption). This is all very approximate as the details matter of how (as in, through what interactions) the adsorbent interacts with the gas molecule. 

In addition, the magnitude of the critical temperature may be considered as a proxy for the strength of intermolecular interactions. For instance, if you assume the gas follows the van der Waals equation of state, then the critical temperature $$T_\mathrm{c} = \frac{8a}{27bR}$$ where $a$ is a measure of the strength of attractive interactions. You can find other ad hoc justifications for associating higher $T_\mathrm{c}$ with stronger strength of adsorption. For instance, larger, more polarizable molecules will tend to have higher boiling points and critical points. Such molecules will also adsorb more strongly. However the argument that the rate of adsorption increases should be considered carefully, since steric effects may be associated with larger molecules.

Regarding the point that "we want gases to be adsorbed", well, the point is that you want to predict behavior. The book merely arms you with guidelines that allow you to make predictions. So that, for instance, if you have a number of compounds in a gaseous mixture, you can estimate which adsorb more. Or perhaps you can use that ability to select new substances with desirable properties based on the adsorption behavior of other compounds.