text: Cables stretch. Shifting becomes just a little bit slower and sloppier and braking starts to be less responsive. Wheels are the biggest items that need retensioning. Also bearings on the bottom bracket, hubs and headset can become sloppy and be damaged if not optimally tightened and lubed. Little things like handlebars and seatposts and saddles that are just out of adjustment can be tightened and straightened out. Plus, they usually give the bike a good cleaning which makes it work better. A more expensive bike built with very good parts may not need everything done as often but more modest bikes definitely need a good going over every year. Winter is an awesome time to get these things done.