Zodiac is reference plan in astrology. Without Zodiac we can’t accurately map the placement of planets from the earth.
If you are talking about Sun sign then it has less influence on our personality. Let me explain the difference between Sun Sign, Moon Sign and Rising Sign to understand this concept better.
Sun sign is the sign (or Constellation) in which sun can be seen from the earth at any point of time. Since sun will be seen in same constellation for ~30 days, any two people born within 30 days of sun transit between constellations (Sun will be in same position, since earth is moving around the sun we will be observing sun transit) will have same sun sign. Let me explain with an example, everyone knows that earth takes one year to complete one revolution around the sun. Since our point of observation is from earth, sun will be seen in 12 different constellations during 12 months.
Moon sign is the sign (constellation) in which moon seen from the earth at any point of time. Moon takes ~28 days to complete one revolution around the earth, so moon will be viewed in same constellation for ~2.3 days. Any two people born between ~2.3 days between moon transitions will have same moon sign.
Rising Sign (Ascendant):
Ascendant sign is eastern point of the sign (Constellation) viewed from the earth at any point of time. Earth takes 24 hours to complete its self revolution, so eastern point of the sign viewed from earth will vary for every ~2 hours (24 hours/12 Constellations). From this it is logical that rising sign is depend of time of birth of the person and hence astrologers give much importance to rising sign in predicting life events of individuals. Next logical question that arise in our mind is why our ancestors choose sign that rises on eastern point as rising sign? Think what you see every day in the east. Will you be able to relate?
Sun Sign will be same for all born on same month, Moon Sign will be same for all born on same day where as rising sign will be unique based on individuals time of birth.
Let me explain with a analogy, Sun Sign & Moon Sign predictions are like taking Paracetamol for fever, it will work out at times if it is general ailment, whereas Rising Sign prediction are like taking antibiotic based.