When the Apple ][ launched, it was marketed first in the USA. It was one of the first microcomputers to have built-in video circuitry for direct connection to a television monitor.
The television encoding was, as you might expect, the US NTSC standard (Incidentally, NTSC was mocked in Europe where it was said to stand for Never Twice the Same Colour). This meant that the machine could not connect directly to a European television.
Before Apple officially launched in the UK, ITT marketed a licensed copy - the ITT2020. This had a separate PAL (Phase Alternate Line) encoder card in slot 7. This card provided the UHF output for the television. Unfortunately this also had the side effect of altering the memory configuration and preventing some software running. PAL is 625 lines while NTSC is 525 lines, so extra memory was required for the graphics.
Apple shortly afterwards launched the Apple ][ model in Europe, including the UK. This official version had a different PAL encoder in slot 7 which altered the memory layout less, allowing more software to run correctly. I understand that a SECAM encoder was similarly provided for the French market, although I never saw one of these.
For example, I recall dumping my ITT2020 and getting an Apple ][ so that I could run VisiCalc.
Slot 7 was reserved for video cards. As @dirkt has already answered, it was unique in providing the video SYNC (pin 19) and COLORREF (pin 35) signals. SYNC was vital to provide the flyback pulse of the TV signal. If these signals had been daisy chained onto other slots, the degradation in their waveform would have resulted in a poorer quality picture. Any weakening of sync would have introduced jitter, or worse, into the viewed image. Also, at the time, the thought of wanting to drive anything other than one single TV was not considered viable so no effort was spent maintaining these signals onto other cards.
In time, third party suppliers did develop higher resolution video cards for slot 7.