All reactions you gave are typically used in protein labelling, e.g. with fluorescent probes.
In order to allow further measurements on the labeled protein, the following conditions have to be met:
The bonds formed have to be stable like the peptide bonds in your protein. You don't want the probe flying around somewhere.
Proteins tend to be a bit sensitive to degradation; boiling in acetic acid is obviously not an option to run the labeling. As a consequence, the functional group to be attached needs to be activated, e.g. with good a leaving group, to allow the labelling of the protein under mild conditions. The succimidyl ester in the first reaction is a good example.