Indoor track isn't just outdoor track under a roof or in a bubble. Not only are the track and events different from those contested outdoors, but these differences make indoor track attractive to track and field athletes for a number of reasons.

Different Track Size
The most obvious difference between indoor and outdoor track & field is the size of the track:

Shorter. A standard indoor track is 200 meters around, or exactly half the size of a standard outdoor track, but unlike outdoor tracks, indoor tracks vary in length depending on the floor space available.

Some are smaller (the track in Madison Square Garden, home for many years to the indoor National Championships and to the Millrose Games, the oldest invitational indoor track-and-field meet in America, is only 160 yards around.)
Some newer venues are large enough to accommodate oversized indoor tracks as long as 300 meters.

Fewer running lanes. Most indoor tracks have six lanes, instead of the eight or nine lanes typical of an outdoor track.

Banked turns. Another difference between indoor and outdoor tracks is that the better indoor tracks will have banked turns. They are bowl shaped rather than flat on the turns. In the same way that your car would slide to the outside going around corners when you drive if you didn't slow down as you begin your turn, athletes going at high speeds would experience the same thing. On flat indoor tracks with tight turns they either must slow down to negotiate the turns, or be pulled outwards. The banks help the athletes cope with these centrifugal forces so that they can maintain their speed on the turns.

Different Events
Due to the smaller track and limited area for field events, the events contested indoors are different from those contested at an outdoor track and field meet:

Fewer events. The full complement of 20 outdoor events each for men and women (excluding relays) is reduced to 12.

Events dropped: The following events contested outdoors are not contested indoors:

100 meter sprint
100 meter hurdles
400 meter hurdles
Steeplechase
10,000 meters
Racewalk events
Discus
Hammer
Javelin

In addition, the world and national indoor championships have recently dropped the 200 meters from their schedules after concluding that the tight turns gave an undeniable and unfair advantage to the runners who competed in the outside lanes (lanes five and six). However, the 200 meter event is still contested at many invitational, collegiate and age-group level meets.

 Mick Priest of Fr.Murphy A.C. competing in the High Jump during the Irish Life Health Masters Indoors Championships at AIT in Athlone, Co Westmeath

Events added. While some events are dropped when the sport moves inside, several events are added to the indoor line-up:

50, 55 or 60 meters dash(to replace the 100 meter dash)
50, 55 or 60 meter hurdles (replacing the 100 meter (women/girls)/110 meter hurdles (men/boys).
3,000 meters (frequently replacing the 5,000 meters)

Indoor weight throw (to compensate for the omission of the three outdoor throws; it is most similar to the hammer event, but because it is heavier than the hammer, it doesn't travel as far and this makes it suitable for an indoor arena)

Pentathlon (five events over two days) (replacing the outdoor decathlon (men) and heptathlon (women). Sometimes in a one-day event there might be a triathlon.

A chance to have fun
Because of the variation in track size and events, indoor track is attractive to track and field athletes for a number of reasons:

It allows athletes a chance to be creative with their event choices and to try a new event or a different event from their specialty,
It provides athletes the opportunity to work on a particular aspect of their main outdoor event:

Sprinters love to use the shorter distance to work on their start.
400 meter hurdlers benefit from racing the shorter hurdles used indoors to perfect their technique.
Distance runners like to race a longer event to build strength or try a shorter distance to work on their speed.