Ltd (for limited) or Inc. (for incorporated) are legal synonyms. It means a legal entity that is to be considered indepently of its owners (i.e., liability is limited to the company, it has to produce its own income tax report, ...). It costs about 1000$ to create a corporation, but the real burden is that you have to do a lot of paperwork ona recurrent basis (income tax, salary reports, ...). Typically, this is appropriate for yearly revenues over 100-150k$.
Enr. (for enregsitré or "registered" if you prefer) is the equivalent of a "banner to do business". It does not create a legal entity (i.e., the owner has to include revenues and expenses of the business as part of his own personal income tax report). It costs about 35$ to register.
You may register with no special commercial name (i.e., the equivalent of saying "I, Joe Smith, declare that I will do business in the District of Montreal") or you you may register with a commercial name (i.e., the equivalent of saying "I, Joe Smith, declare that I will do business in the District of Montreal under the name "Massage ACME enr."). Enr. can be used by an individual alone.
Strictly speaking, you HAVE TO register in order to do business when you do over a certain amount of money per year - over 30k I believe (so babysitters, ... are out of this!). You do not have a choice.
You do not have to put the "enr." (or the "inc." for that matter)everywhere in your communications (in legal documents you have to, though). It is a matter of choice (e.g., IBM, Microsoft, ... do not use the "inc." in their ads, but they could if they wanted to).
As far as "why do they bother mentionning it in the ad?", "do customer care?", ... Customer may care in certain business domain (consulting, ...) but I believe that in the business of escorts and massages, it is not as if customers really cared! . I guess they just do mention it in the add as an habit, not with specific intentions. That's what I think anyway.
Well, I was told very recently by a masseuse that the Journal de Montreal and LaPresse require that escorts or Massage Parlours be a company of some sort (registered or inc) to place an ad. My source also mentionned to me that to advertise in Lapresse for Massage, you must present a proof of Massage License before the ad shows!! Hmmm...licensed? And by who??