Why need i buy Galletto 4, Kess v2 and Ktag all together?

Because non of China tool will cover all cars good and if you buy clone tools, buy all (galletto+kess+ktag) from them, that you will cover more cars, or better to buy a good original tool if you will do the work as a pro, all depends want you will do.
Reference:  Kess v2 v2.22 vs Ktag v2.23 vs Fgtech v54 Galletto 4

Product	Kess V2 V2.23	K-tag V2.23	Fgtech v54 Galletto 4
Function	read and write the Data of Engine ECU	read and write the Data of Engine ECU	read and write the Data of Engine ECU
Support  Vehicles	Cars and part Bikes	cars, trucks,traktors and bikes	Car, Truck, Motorbyke, Marine
Support Protocols	With coverage of all major brands and protocols including Line, CAN, EDC17 and MED17 and Ford J1850	Available Protocols for K-TAG: BDM MOTOROLA MPC5xx	NEW JTAG – BDM – MOTOROLA – FREESCALE – INFINEON
Method	Via OBD2	Via BDM	Both via BDM and OBD
Update	Update via Hardware	Update via Hardware	NO
Token	No tokens limitation	No tokens limitation	No
Language	English, German, Spanish, Italian, Portuguese, French	English, German, Spanish, Italian, Portuguese, French	English, Italian, German, French, Spanish, Portuguese, Czech, Turkish
EDC17	work with a small part	work with a small part	work with a small part
Additional remark	1)  Most customers use FGTech Galletto 4 V54. Some users prove fgtech2 v52 works 50% obd. BDM works ok and work ok with tricore and st10 micros. Fgtech galletto not disappoint them never. 2)   KESS V2 has Anti-mediation lock module, it can do new and encrypted cars better than Fgtech. Remark: no one is perfect, maybe is good to have more than one tool, but much better is to have one good tool.