Ubuntu – Iptables: Difference between RELATED and ESTABLISHED state

I've tried reading many articles on the internet but none of them are quite clear. I also know similar question has been posted here before but none of them explain my situation. Earlier I ended up wasting 2-3 hours because of this.

Manual says:

NEW — meaning that the packet has started a new connection, or otherwise associated with a connection which has not seen packets in both directions, and

ESTABLISHED — meaning that the packet is associated with a connection which has seen packets in both directions,

RELATED — meaning that the packet is starting a new connection, but is associated with an existing connection, such as an FTP data transfer, or an ICMP error.

My iptables on Server look like:

iptables -P FORWARD ACCEPT
iptables -P INPUT ACCEPT
iptables -P OUTPUT ACCEPT

# Flush old rules
    iptables -F

# Loopback
    iptables -I INPUT 1 -i lo -j ACCEPT

# Allow responses from OUTPUT connection
    iptables -A INPUT -m state --state ESTABLISHED -j ACCEPT

# Apache
    iptables -A INPUT -p tcp --dport 80 -m state --state NEW,ESTABLISHED -j ACCEPT
    iptables -A INPUT -p tcp --dport 443 -m state --state NEW,ESTABLISHED -j ACCEPT

iptables -P FORWARD DROP
iptables -P INPUT DROP
iptables -P OUTPUT ACCEPT

Whenever I try curl --verbose https://server_ip/ from my PC, it works perfectly. But when I tried this from inside the server after SSHing into it, it always stuck at CLIENT HELLO. Why it is happening? Note: curl --verbose http://server_ip/(without SSL) worked perfectly from both local PC and server.

After I executed iptables -A INPUT -m state --state RELATED -j ACCEPT, it started working from inside the server too. Again why it started working?

Manual says RELATED -- meaning that the packet is starting a new connection. Is it a security risk? Can a client open a connection to a different port using this? When it is useful?