With effect from 31 March 2017, there is no need to use the company seal to execute legal documents under Section 41 of the Compaines Act.
Section 41A of the Compaines Act states explicitly that it is not a MUST for company to have a common seal.
Section 41B of the Companies Act states that a company may execute a deed without a common seal by signature:
On behalf of the company by a director of the company and a company secretary;
On behalf of the company by at least 2 directors of the company; or
On behalf of the company by a director of the company in the presence of a witness who attests the signature.
Where a document is to be signed by a person on behalf of more than one company, that person has to sign the document separately in each capacity. If not, it will not be considered signed.
In summary, the company seal is being phased out in Singapore. If another one tells you otherwise, either they are not updated on the Companies Act or trying to earn more money from you as their service provider.