Semi-colons can be used before a conjunction like 'and' or 'but' to join two simple sentences, one or both of which already contain commas. The semi-colon is used in this way to outrank the commas. e.g. Nowadays, Brighton is a popular destination for Londoners to travel to the coast for a day trip; but, it was in the 1700s that Brighton became a popular destination for people to drink and bathe in seawater.
This use of a semi-colon before a conjunction when joining two sentences is a slightly outdated practice and it is more common for writers to use a comma. If there are no commas in the sentences then a semi-colon should not be used.