I use PETG when I want the object to be somewhat rigid, but flexible enough not to break, especially object having thin parts. PETG is strong enough to print two layer 0.5 mm thick and be very flexible in the z direction, such as printing straps.
Basically PETG is amorphous while PET/PETE is crystalline. Amorphous gives a slightly lower melting point and less out-gassing, characteristics better for 3D printing. PETs crystal structure makes it more rigid than PETG, but leaves PETG more impact resistant and easier to print. The more rigid PET translates to more dimensional accuracy as with PLA.
PETE (or PET)
PETE is the most common variation of the thermoplastic polymer resin. The clarity of PET is considered to be the best of all plastics with very good chemical resistance. PETE is formed into containers via 1-Step or 2-Step Injection Blow Molding.
Polyethylene Terephthalate Glycol (PETG) is an amorphous variation of PET. PETG cannot be oriented and therefore you will not find pre-forms of this type of resin. Extrusion, Injection Blow Moulding and 1-Step Injection Stretch Blow Moulding are all possible for PETG, it is also used frequently for 3D printing. Clarity and chemical resistance are considered good when compared to the previous three variations, however PETG has poor impact resistance.
Why is PETG used for 3D printing?
Quote from https://www.3dnatives.com/en/petg-3d-printing-guide-181220194/#!
The characteristics of PETG
PETG is therefore a copolymer, combining the properties of PET and glycol. The addition of the latter reduces the overheating issues of PET and therefore its brittle appearance. Among the main characteristics of PETG are its hardness, impact and chemical resistance, transparency and ductility. It is an easily extruded material with good thermal stability. It is particularly appreciated for its food compatibility. On the downside, note that it requires a heating plate to avoid the warping effects found in ABS 3D printing – even if the warping rate is low, it is better to use a BuildTak sheet to make sure the material grips. It is also more prone to scratches than PLA. Finally, it can quickly take moisture and will keep better in a cool and dry environment.