I kind of have to disagree with what user mossadrai explained about sect, I do think it is of great relevance for astrology today.
Sect is based on the concept of day(masculine) and night(femenine) as mossadrai stated, and its one of the core principles in which astrology is founded. This principle classifies signs into 2 distinct sets: diurnal signs (fire and air) and nocturnl signs (water and earth).
- Diurnal planets (Sun, Jupiter, Saturn) are by default out of sect in every nocturnal chart.
- Nocturnal planets (Moon, Venus, Mars) are by default out of sect in every diurnal chart.
- Mercury belongs to the diurnal sect if it rises as a morning star, and of the nocturnal sect if it sets as an evening star.
You may also notice that diurnal planets rule diurnal(male) triplicity signs, and nocturnal planets rule nocturnal(female) triplicity signs. Sect in itself derives into the reason as to why "trines" and "sextiles" are generally seen as "easy" aspects because they unite planets in signs of the same sect.
The part about the in-sect planets being above the horizon is the explanation to the concept of Hayz, in which a planet is better suited when above the horizon, and in a sign of its own sect. The implication is that the planets can express its qualities better this way, because the 7th-12th axis is connected to the exterior world. Consider that most of the more powerful and weak houses are all on this axis except for the "1st" house (7th/10th/11th an 8th/12th). Being in a sign of its own sect also makes the qualities of the planet better expressed, because the planet combines well with the sign.
I consider the principle of sect of great importance in chart reading even today. It provides information relevant to the planet.
For example, when we judge a problematic chart we always look at the out of sect malefic, which has the most potential for trouble on the chart, Saturn for night charts, and Mars for diurnal charts. The same applies for the benefic planets when looking for favourable placements, with Jupiter being particularly benefic in diurnal charts, and Venus being particularly benefic in nocturnal charts. A trine or sextile from a planet to your in sect benefic is considered a very good placement. A square or opposition to your out of sect malefic is considered a very bad placement.
It also helps in horary judgement, when properly used, but it derives extra information about the question and the state of the planet. It is also used for temperamental analysis in birth chart reading, for elective astrology, etc.
Personally I do use the concept of sect, heavily in birth charts because it is the core stone of astrological principle. Pretty much every astrological concept is derived from sect.