The main reason for the merger between the two was so that foreign Maestro cards could be accepted in the UK. Despite the merger gambling games, there are transactions such as the Point of sale that is still handled by Switch Card Services. Retailers and casinos who accept the Maestro card will only accept UK issued cards that were former switch cards.
Maestro/Switch cards that are issued by banks such as the HSBC group have an issue number that can be seen at the bottom of the specified card. The number corresponds to how many times the debit card has been issued. The card number will not be easily changed if the card expires or it gets lost.
The switch is still used on some e-commerce websites despite the fact that it has not been a brand for quite a number of years. When they use the term switch they are talking about the Maestro debit cards.
The switch used to be used to purchase products that would be later debited from your account (usually used to take two days). Most credit cards you would have to be billed at the end of the month unlike the switch debit card. When using the switch debit cards you would pretty much use it in the same way as other credit cards to fund your online casino account.