Softwares are generally updated. As developers find bugs and develop new features, a software is updated with bug-fixes and these new features. But not all updates are liked by people. Sometimes when software gets updated, people find it less usable than the older versions and prefer to remain with the older versions. however these older versions disappear over time and we have to change to a new a software or are forced to use the new version. for instance, Nero 6 was very light weight & had all features one would want. But with subsequent versions it suffered a feature overload & the size kept on growing. Now it takes over a GB on installing. flashget was adware free till 1.96, but version 2 displays ads. So, sometimes you might prefer to stay with the older version.
However, the older versions are not all lost. Some sites offer old versions of softwares for download. If you want the older versions of any software, you can visit the following sites:
One of my personal favourite mobile software is opera mini. there may be 100s of reasons you might want to stay with an older version. so here you can get all old versions: http://my.opera.com/community/forums/topic.dml?id=221224