Advantages

1. This is the last hope for candidates who have not been allotted any seat in previous rounds.
2. Every candidate ( both who is allotted a seat in any NIT, IIIT, GFTI and who hasn't been allotted) have the chance to fill their choices again if they aren't satisfied.
3. This is the final chance for students to even change their branch or institute if they aren't satisfied.
4. It is beneficial to government as less no. of seats are left vacant than before hence less fund wasted.

Disadvantages

1. For spot round, Candidate has to be physically present at the desired institute for physical reporting. This is quite impossible for people living far off from the institute.
2. Now the seat allocation is not according to merit. It is like first come first serve basis. So not worth it.
3. You can't get refund of participation fee in spot round once you pay.
4. Getting a seat in spot round is not guaranteed and you may lose your candidature for JEE next year.

Seeing the above disadvantages, CSAB came up with a special round. In this special round instead of spot round, counseling was done online according to merit of the applicants. So this became an advantage for both the government and the candidates.

Central Seat Allocation Board will be conducting CSAB 2018 for admissions into NITs, IIITs and CFTIs through different channels. CSAB 2018 special round for NITs, IIITs and CFTIs will be held for those candidates who were not allotted in JoSAA counselling but have valid JEE Main scores. The registration for this channel will commence tentatively from the fourth week of July 2018. Candidates belonging to Northeastern states and Union Territories will be able to apply for admission through CSAB NUET 2018. The application form will be available tentatively in the first week of July 2018. Allotment of seats will be done according to the candidates’ merit and preference of seats. Similarly, CSAB UT and SFTI will be conducted for Union Territories candidates and Self Financing Technical Institutes (SFTI) accordingly. Around 19 institutes are currently participating in CSAB 2018. 

Visit this link for more details

https://engineering.careers360.com/articles/csab