1. Increase strength on the ball to resist challenges and hold the ball up.
2. Increase vertical jumping ability (i.e. heading and aerials challenges, GK saves).
3. Increase speed and acceleration/deceleration.
4. Injury Prevention.
5. Increase power of kicking and striking (i.e. Clearances and Shots).
6. Improve SAQ (Speed, Agility, Quickness)
7. Improve the endurance of strength specific activities in soccer.

Strength training (as in any other sport) should be implemented in a safe and gradual manner.  This progression also aids the athlete to peak at the correct point in time.  Strength training in note the main focus of overall training (perhaps 1-3 sessions a week, depending on the time of the season).  However, it has a key place in particular in pre-season.  Strength training should be developed for football in a pyramid based model in the 3 phases below.

Each of these phases should be a 2-4 weeks long.  Ideally 2-3 session/week.

1. Foundation (Weeks 1-2) - This phase focuses on the preparation of the muscles, tendons and ligaments prior to increased resistance.  It has several purposes:  This phase should be used to develop correct exercise and movement technique.  Strengthen and develop underlying stabilizer muscles.  Create balance within certain muscle pairs (i.e. hamstrings/Quads) and also even out strength imbalances between the left and right side.  This phase is characterized by high reps (12-15) and low resistance (weight).  30-90 seconds rest between sets.
2. Maximal (Weeks 3-4) - The maximal phase is designed to increase maximum strength of muscles and muscle mass development.  This phase is also where we look to optimize the development of muscle force and power.  This phase is characterized by increased resistance (weight) and lower reps (6-8).  30-120 seconds rest between sets.
3. Endurance (Weeks 5-6+) - This phase is to develop resistance to fatigue for the strength related activities that are performed in a soccer match.  Players should be able to perform strength activites multiple times during the duration of a match.  This phase is characterized by high reps (12-15+) and low resistance (weight).  30-90 seconds rest between sets.