I've noticed while looking through presidential candidates for the 2016 election websites that they all have "Paid For ..." in the footer of their websites. Obviously they paid for it, but is this a marketing technique or is it something political? I am just curious as to why do they all do this.
In the United States at least, this comes directly from the FEC:
What disclaimers must appear in political ads and mailings?
Political committees must include a clear and conspicuous disclaimer on all "public communications" (e.g., TV and radio ads, newspaper, magazine or outdoor advertising, and mass mailings or telephone banks to the general public). Public communications financed by individuals or other organizations must include a disclaimer if the communication expressly advocates a candidate's election or defeat or solicits funds. The disclaimers must indicate who paid for the communication and whether or not it was authorized by a candidate. Unauthorized ads must also include contact information for the sponsoring organization.
The reason being that, if I were a nefarious individual with gobs of money, I could knowingly broadcast false and misleading information and there would be no way the public could weigh my or my organization's credibility against the claim.
You can read more about the FEC rules regarding this from this online brochure (pdf).
In the context of television broadcasts, the FCC enforces 47 USC § 315 which states:
(C) Television broadcasts
A candidate meets the requirements of this subparagraph if, in the case of a television broadcast, at the end of such broadcast there appears simultaneously, for a period no less than 4 seconds—
(i) a clearly identifiable photographic or similar image of the candidate; and
(ii) a clearly readable printed statement, identifying the candidate and stating that the candidate has approved the broadcast and that the candidate’s authorized committee paid for the broadcast.
This section of the statute is more about limiting how much a candidate can be charged by those providing the platform for the advertisement, and I include it only to show that they include that message in the ad to satisfy this qualification.
I'm not certain there's any regulation or law that requires a candidate to specifically put this information on their own website, but even if there isn't the public in the US (in my opinion) now expects that type of message to be present on most if not all political advertisements of any sort.