Over the last 130 years logos have evolved from detailed, in your face, you will never forget me to a more elegant, stylised, modern and simplified logo. Where the company is especially well known, for example Pepsi, Xbox and BP have removed the entire wording from their logos and it is nowmade up of a coloured design. Obviously only companies that are confident that their customer will know them by their colour and design can do this.
Why would a company change their logo? Well just as the company evolves, so should the logo to ensure it stays relevant to the company and is not out-dated. Although there has been a revival of retro branding where companies use their original logo to show they are an established brand and let the customer connect with their history.
Also using a more simplified logo makes it more versatile to use so it can be used in situations from a business card to advertising banner to the tail fin of a plane
Often the logo will change when a company changes its name or the company has been merged.