We need to understand that evil spirits exist and that's the reality and they operate any day and any time. As a Christian we need to understand spirituality controls the physical and God has given us authority over principalities and power including evil spirits using the word of God as a backing against them.
The utmost idea behind evil spirt is that they don't want Christians to fulfil their purpose on earth. They are sent to stop the fulfilment of God's word in human either by tormenting or causing christains to encounter challenges. When christains encounter those challenges they will begin to doubt God or sin against God or caused fellows to be wicked to others.