Why Dates On Coins?

Without dates, how else would you have found all those vairations of the 1964 dime.

As I recall, dates were far more important when intrinsic metal was involved in coinage. Metal used in the various denominations varies over time and therefore a coin dated in one year may have more or less precious metal than one made either before or after even if the denomination is identical.

Since most collectors are familiar with US coins, a good example are the Seated Liberty 25 Cent coins. As a kid I assembled a type set of these coins. There are several minor design changes, but there are also intrinsic value changes. The government was forced to alter the composition to prevent hoarding of coins. If there was too much silver the coins went into the melting pot - too little they didn't circulate well. So you have several weight changes made during this series which was made from 1839 to 1891. The standard was 0.1934 OZ from 1839 to 1853. This was lowered to 0.1800 OZ from 1853 to 1873. It was raised to 0.1808 OZ from 1873 to 1891. If you notice, both chages occurred in mid-year, so in 1853 and 1873 there are actually two assays for each year. To mark the difference in both cases, the mint added arrows at the date. This created extra types.

Without dates - how would you know the actual metal content?

Cool answer.
Would you know Why and When did they start putting mint marks on coins?

"Mintmarks" predate virtually any other design feature of coins. Indeed, it could be argued that the very first coins from Lydia and other early Greek states were simply blobs of gold, electrum or silver with a mintmark on them - the "mintmark" being a modification of the personal seal of either the king or the early mintmasters. Pictures and monograms were used as symbols for the various mintmasters in many ancient coin series, though these were not truly "mintmarks" in the modern sense.

Alexander the Great's kingdom was the first "world superpower" to issue a standardised coin throughout the empire, but the symbols used to distinguish the coins of different mints are not always easily understandable today, even for the specialists in the field.

If by "mintmark" you mean an easily translatable symbol, such as "D" for Denver or "S" for Sydney, then you'd have to go back to the Romans for our inspiration. In the coinage reform of emperor Diocletian in 296 AD, a system of mintmarks was introduced throughout the empire, replacing the various local coinages used before this. These symbols were simply a group of up to half a dozen letters, usually incorporating an abbreviation of the mint-city. Other letters stood for "money" or for the particular branch mint within that city that was responsible for that particular coin. Some larger cities had over a dozen of these branches.

Some examples:
MOST = Ostia, Italy (here the "M" means "moneta", or money).
SMANTB = Antioch, Syria (here "SM" is "sacra moneta", or sacred money, and the B is branch mint No.2 (B is the Greek number 2)).
PLON = London, Britain (here "P" is "pecunia", another word for money). Australian coins struck in London in the 1950's had the "PL" mintmark - a 1,700-year-old tradition!
AQT = Aquileia, France (here "T" is branch mint No.3, or "tercius").

Many countries/empires since then, that have issued a unified currency throughout the nation, have used mintmarks to denote from which part of the nation the coin originated. France, Germany, Spain (and the Spanish colonies in the Americas) and Britain have all used them to particular effect.