What are the things which can be done with SSL VPN which cannot be achieved with IPSec VPN? What are the reasons to opt for SSL VPN instead of IPSec VPN?
Security wise there's no really big difference, as both IPsec VPN and SSL VPN can use similar encryption algorithms. Some sources like Paul Bischoff's VPN encryption explained: IPSec vs SSL may state that IPsec VPN requires a PSK that could be a security concern, but many implementations actually allows certificate based authentication for IPsec tunnels.
Advantages depends on use case
The key difference is that, as a higher layer protocol, TLS used in SSL VPN can easily go through NAT, whereas IPsec VPN requires NAT traversal techniques, and they aren't always working on all networks. Also, as TLS used for VPN isn't easy to distinguish from other TLS protocols like HTTPS, it's harder to block on firewalls. This makes SSL VPN ideal for client-to-site VPNs.
On the other hand, IPsec VPN (especially IKEv2) can negotiate connections faster, so there might be performance advantages. This performance and reliability perspective makes IPsec VPN ideal for site-to-site VPNs, as they typically don't have to deal with NAT traversal.