Why does Chelsea's goalkeeper Cech always wear a helmet?

Cech wears a hat mainly to protect his head.

Cech suffered a fractured skull in 2006 when he collided with Reading's Stephen Hunt in the Premier League, from which he recovered only through surgery. Although he recovered quickly and returned to action before the end of the season, doctors advised that he would have to wear a brace to protect his head and avoid another head injury.

Extended information:

Petr Cech. Petr Cech, born May 20, 1982 in Pilsen, Czech Republic, is a Czech footballer who plays as a goalkeeper for Arsenal Football Club in the English Premier League.

Cech joined Chelsea in the English Premier League in 2004, where he grew into one of the world's top goalkeepers, winning the Premier League Golden Gloves Award three times and the Czech Footballer of the Year seven times.On June 29, 2015, Cech officially transferred to Arsenal.

Cech helped the Czech Under-21 youth team win the European Under-21 Youth Championship in the summer of 2002, and also helped the Czech national team reach the quarterfinals (once, in 2004) and the last eight (once, in 2012) of the UEFA Europa League.