Most of the monomers are available with containing inhibitors! Is it just to avoid polymerization or something else?
Monomers containing different functional groups in highly reactive form. To protect the unwanted reactivity and also polymerization, the inhibitors were added in monomers.
As stated above most unsaturated compound are prone to polymerization, other addition reactions, oxidation etc. Therefore if you want to maintain the initial purity during delivery, you should add inhibitor. The amount added may depend on the reactivity of the compound. Sometimes the monomers are supposed to be delivered in cooled state - although this is rarely taken into account even by major fine chemicals companies. Until the inhibitor is consumed the monomer is protected.
In some cases the inhibitor is not removed from the monomer only more initiator is added - a part of which is consumed for reacting with the inhibitor. This is not an elegant method, especially if the product is bulk polymerized and if the degradation products are not volatile. More frequently the inhibitor (which is mostly a hydroquinone-like entity) is removed by aqueous alkaline washing (sometimes combined with sulfites) or, if the inhibitor quantity is low enough ion exchange resins with amine-like groups are used. The capacity of the resin should be taken into account. Distillation does not always properly remove the inhibitor.
It is added to avoid undesirable polymerization and side-reaction scenario (like hydrolysis, cyclization etc).
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