It's like an idiomatic expression "guardarsi intorno" = to look around; there are a lot of terms using "guardarsi".
"guardarsi" itself means "to look at" or "to take care"
guardarsi intorno = to look around
guardarsi attorno = to keep yourself informed; but also: to be careful
guardarsi alle spalle = to be on guard; to be wary
Maybe this helps. Some verbs simply are reflexive, without necessarily having an english equivalent reflexive construction. Other languages are similar. For example "to catch a cold" in German is reflexive while in english it isn't. It doesn't always make sense why it's reflexive in one language and not in another. It's just something to learn.
My understanding of it is "guardare" is used as a transitive verb, meaning one that takes a direct object -- one looks AT something. "Guardarsi" I think is used more in the sense of "looking around". I know that doesn't explain WHY it's reflexive, other than to say that in some languages (German e.g.which I'm much more familiar with) some reflexive verbs simply have to be used reflexively and don't necessarily have an English (reflexive) equivalent. So, e.g. "to catch a cold" in German is reflexive. Italian has the same sort of verbs which may always be reflexive, with or without an equivalent English translation, or they may be sometimes reflexive, sometimes not - "guardare/guardarsi" in which case their meaning and use changes, maybe slightly, maybe a lot. Hope this helps.