Nomenclature of complex benzene-based substituents
Intro

Your statement, that complex substituent names are placed in square brackets, is incorrect. These are to be placed in parentheses (round brackets). Correct preferred IUPAC name (PIN)[1] example:


  
  2-(3-methylbutyl)phenol (PIN)


Square brackets are used when the substituent name already contains parentheses, e.g.:


  
  2-[(2R)-2-methylbutyl]phenol (PIN)


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Retained aromatic compounds allowed for substitution

You can see that the names above are based on phenol, which is one of retained aromatic functional parent compounds, with allowed substitution, in preferred names.

The other ones are aniline and benzoic acid (see IUPAC rule P-34.1.2.[1]). E.g.:


  
  2-hydroxybenzoic acid (PIN)
  (not 2-carboxyphenol, as $\ce{-COOH}$ group is preferred over $\ce{-OH}$)


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Retained alkyl-substituted aromatic hydrocarbons (not allowed for substitution in PINs)

There are also retained alkyl substituted aromatic parent hydrocarbons (P-22.1.3):


  
  toluene (PIN)
  methylbenzene


and


  
  1,2-xylene (PIN)
  o-xylene
  1,2-dimethylbenzene


(and the two other xylenes).
But none of them is allowed for substitution in preferred names, e.g.:


  
  1-chloro-4-(chloromethyl)benzene (PIN)
  α,4-dichlorotoluene


Xylenes are not allowed for substitution even in the general non-preferred names:


  
  1,2-bis(bromomethyl)benzene (PIN)
  α-bromo-2-(bromomethyl)toluene
  (not α,α′-dibromo-o-xylene)


(Yet another retained name mesitylene for 1,3,5-trimethylbenzene (PIN) is allowed only for general names, not allowed for substitution.)

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Complex aromatic substituents

Now if a complex substituent is made from substituted phenol, aniline or benzoic acid, the substituent name is no longer based on these names. They can be thought as suffixes, but now we have another higher-priority “suffix”, -…yl. E.g.:


  
  3-hydroxy-4,5-dimethylphenyl acetate (PIN)
  (not 2,3-dimethylphenol-5-yl acetate)


(In case you didn't hear ‘phenyl’ yet – it's not to be confused with ‘phenol’, it's the only, notorious, name for “benzenyl”, $\ce{-C6H5}$)

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References:


Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book)